Okay. did you see my earlier guesses? You haven't responded to those ones (judging from the time stamps they were probably posted when you were replying to zuckyd1).

Andy E. Nystrom wrote:

Loki wrote:

All questions pertain to handbooks or handbook-like (Western Files, Civil War: Battle Damage Report, etc.) published by Marvel - so no DC's Who's Who or Dorling-Kindersley guides.

1. What alien race was the first to get profiled in the original Handbooks' Alien Races section?
2. What was the first alien race in the Alien Races' section to be depicted fully naked (e.g. not wearing a black thong/codpiece)?
3. In what publication did it first become the rule rather than the exception to include headshots for characters depicting (where available) their unmasked/powered down/untransformed faces? 
5. Some lines in the character bios have been merged or slightly renamed, but one has been dropped altogether since the original handbooks. Which one?
6. Which handbook entry was the first to introduce new information first revealed in that handbook and not gleaned from a prior comic story?
9. A fairly general one - why did the 21st century handbooks abandon using the Roman numerals?
12. One publication had a single entry massively out of alphabetical order. Which entry?

1. Aakon

Correct.

Andy E. Nystrom wrote:

2. Brood?

Yep. Steal your body and life, and no sense of modesty either. They started a trend for nudity among the aliens, as the very next page the Chr'ylites, Ciegrimites and Cotati all went au naturel too.

Andy E. Nystrom wrote:

3. A-Z Hardcovers?

Nope.

Andy E. Nystrom wrote:

5. Marital Status

You might think so, but not quite. Marital status was effectively merged into known relatives, as spouses are named as such. No spouse mentioned means the character is presumed single. There's another heading that has been dropped completely.

Andy E. Nystrom wrote:

6. Alpha Flight at the very latest; Marina and Puck joining

Even earlier.

Andy E. Nystrom wrote:

9;. Further to what zuckyd said, retcons (some characters first appeared earlier chronologically while others first appeared earlier by publication date). Plus really obscure characters keep turning up, and some people don't like it when Roman numerals are for a completely different character (e.g. most Hobgoblins are Green Goblin variants but not the Imperial Guard member)

Close enough. Basically, the numeral system is fraught with problems, for all the reasons listed. Heck, if you allow for slight spelling variations, Peter Parker isn't the first Spider-Man, because there's a Golden Age villain called Spiderman. And even if you go by strict spelling, Adrian Toomes isn't the first Vulture, as there were at least five Golden Age Vultures. So obscure older versions mess up the numbering system, as you said. It all boils down to: using a numbering system only works if EVERYONE is working out the system the same way and knows about every namesake. Otherwise when you say "Vulture II" you mean Blackie Drago, but I think you mean Ottakar Meltzer. Giving the civilian name (or another identifier if the civilian name is unrevealed or two or more versions also share civilian names - looking at Nighthawk for example) is less ambiguous. 

Andy E. Nystrom wrote:

12. Probably the biggest is New Mutants below X-Men

Arguably right, though they were considered junior X-Men when the idea was developed. But it's not the one I'm thinking of, who is way further out of alphabetical alignment, and not so they could be tied to a related character or group.
 

Andy E. Nystrom wrote:

I think I remember the answer to 16 now: Captain Marvel: Mar-Vell in 1960s-1970s, Monica Rambeau in 1980s, and Genis-Vell in 1990s.

Correct.

Sidney Osinga wrote:

Loki wrote:

10. A two parter, as both are related to the same character. Which handbook volume first acknowledged that Captain Britain had appeared in a British comic before he appeared in an American one? And which volume actually provided readers with that British first appearance info?

Deluxe Edition #2 for the first part and Update '89 #1 for the second?

Correct. Deluxe noted "First American Appearance", thus implicitly noting he had an earlier non-American one; Update '89 actually said what it was.

Sidney Osinga wrote:

Loki wrote:

12. One publication had a single entry massively out of alphabetical order. Which entry?

Armin Zola in the first issue of the Deluxe Edition

Not the one I was thinking of, but I can't say you are wrong. Because they treated his real name like an alias, he got listed under A rather than Z. However, the answer I was aiming for is still up for grabs.

Sidney Osinga wrote:

Loki wrote:

13. One entry for a team depicted members of said team not shown in the comics, and members who we had met in costumes they'd not previously been depicted in. Which entry? Note: I'm looking for a team; don't get mixed up with the answer for the next question.

the Sinister Six?

Nope. More obscure than them.

Sidney Osinga wrote:

Loki wrote:

17. Still Legacy: Three of the books featured different characters who shared the same codename; one doesn't, but does include a character whose codename is a synonym of the one used in all three other books. Which codenames?

The Sun Stealer in the 1960s one and the three Star Thieves in the other 3

Correct.

Sidney Osinga wrote:

Loki wrote:

Added: 18. cf questions 7 and 8 - which Appendix entry first used Roman numerals to distinguish between two characters using the same codename?

Black Knight. Just guessing

Nope. Earlier than that for the Appendix using Roman numerals.
 

12. Wolverine in the OHotMU Wolverine 2004?
15. the Defenders

zuckyd1 already got 15 I believe.

7. 20th century handbooks distinguished between characters using identical codenames with Roman numerals. Which entry (full entry, not Appendix profile) was the first to include the Roman numeral in the profile title?

Red Guardian II I think. Book of the Dead and the inactive, and two Red Guardians in a row.

8. And which entry was the first to have the Roman numeral used in the contents page? NB - may or may not be the same answer as 7.

Black Knight (Sir Percy)?

14. Which publication depicted across multiple entries multiple characters not previously shown in the comics for the first time?

While outside the Marvel Multiverse, since it was published by Marvel I'll guess GI Joe: Order of Battle.
 

Okay, revised guesses for 7 and 8, without looking at my copy.
7. Red Guardian I?
8. Since the Air-Walker Automaton is sometimes treated as a separate character (though less so nowadays), the contents might say Air-Walker I-II for the Air-Walker entry.
 

8. I just remembered: Baron Zemo I is likely a good candidate

Answers.

Loki wrote:

3. In what publication did it first become the rule rather than the exception to include headshots for characters depicting (where available) their unmasked/powered down/untransformed faces?

New Avengers: Most Wanted Files. As a more-or-less in-universe document discussing escapees from the Raft, it made sense that the civilian identities be shown - can you imagine in the real world if someone escaped prison and the authorities only provided pictures of the criminal wearing what amounts to a disguise? Though iirc it took us a little longer to think it through (e.g. there were a couple of subsequent "regular" handbooks that didn't do headshots), NAMWF was the first to do so for all applicable entries, and the one that eventually led to us doing it for every handbook.

Loki wrote:

5. Some lines in the character bios have been merged or slightly renamed, but one has been dropped altogether since the original handbooks. Which one?

Base of Operations. Powers, strength levels, unusual features, etc. all got folded into "Abilities/Accessories" and Marital Status into Relatives, but Base of Operations got dropped completely.

Loki wrote:

6. Which handbook entry was the first to introduce new information first revealed in that handbook and not gleaned from a prior comic story?

Abomination. It might be argued that this was a trick question, but it isn't - people tend to think of the entries where big bits of info were introduced, but really new info was being provided from the first entry, such as Place of Birth. 

Loki wrote:

8. And which entry was the first to have the Roman numeral used in the contents page? NB - may or may not be the same answer as 7.

Red Guardian I. The Black Widow's ex-husband got an entry as he was believed dead at the time, and the very next entry was the then-inactive (and inaccurately named) Red Guardian II. Obviously he recovered from his mild bout of death, she changed her codename, and retcons revealed at least one earlier Red Guardian so the numbering is wrong, but nevertheless, he was the first to appear in the same handbook as another character using the same codename, and so the first who got numbered in the contents listing on the inside front page.

Loki wrote:

11. Diablo's original handbook entry listed his place of birth as Saragossa, Spain. His more recent entries have changed this, but not because of any retcon shown in the comics. So why has it been changed?

Because he was born in the 9th century, and there was no country called Spain back then, nor was it Saragossa. So it's been changed to Saraqusta, Umayyad Caliphate (modern-day Zaragoza, Spain)

Loki wrote:

13. One entry for a team depicted members of said team not shown in the comics, and members who we had met in costumes they'd not previously been depicted in. Which entry? Note: I'm looking for a team; don't get mixed up with the answer for the next question.

X-Force. No, not the former New Mutants reforged by Cable, nor the guys who then became X-Statix, but the M Branch namesakes who turned up in Cloak and Dagger. In the comics we only saw three of the seven members in costume (four if you count a small subsequent appearance in Fred Hembeck's page in Marvel Age), and headshots of the rest. For their handbook entry, Gus Vazquez drew the entire team in costume.

Loki wrote:

14. Which publication depicted across multiple entries multiple characters not previously shown in the comics for the first time?

Encyclopedia Mythologica - all the pantheons got illustrated, resulting in the first on-panel depiction of a lot of gods. 

Loki wrote:

Added: 18. cf questions 7 and 8 - which Appendix entry first used Roman numerals to distinguish between two characters using the same codename?

Ape-Man I of the Ani-Men, in the first handbook issue. 
 

1. Orka Dorka Doo
12. Mordred

5. 1602 AD
6. the Mad Thinker's in Master Edition #8?
9. No-Girl?
13. if the answer is supposed to be the Cool-meter, then it was a 90s feature in the Nineties Legacy book

7. It the Living Colossus

7. the Golem

Calling a day on the following:
2. Who is the subject of the Marvel Value Stamp in Marvel Legacy: 1970s?
3. What is the name on each side of the sheet for the Master Edition profile on the Frankenstein Monster?
4. In the Master Edition Avengers Membership Roster, what is Vision inadvertently listed as?
6, What is the special coverage at the end of the first hardcover?
8. Which profile in which book has a brief description of Android Andy with connecting image?
9. What is Martha Johannson listed as in the X-Men Encyclopedia?
10. What is the Glossary called in the Master Edition?
11. What was the name of the editorial section in the earliest issues of Update '89?
14. Which editorial piece title that's a play on a popular TV series was used in three different handbooks in some form or another?
15. Who was the main dust jacket penciller on the hardcovers?

I'd held off answering some of these because I worked on the relevant books, but since we're a day off with no answers:
2. Forbush Man
3 Front - Frankenstein, back - Frankenstein's Monster
4 Doctor Pym
7. Crooked World (Earth-238) in Alternate Earths