1. 10K and Lt. Roberto Warren appeared together in episodes of Z Nation (an Asylum/Syfy production).
2. Roberta Warren (then only a sergeant) and Fin Shepard both appeared in Sharknado 3: Oh Hell No! (also from the Asylum and Syfy), in which a sharknado threatens Washington, D.C.
3. Betty Cooper and Veronica Lodge were caught in that same D.C. sharknado in Archie vs Sharknado #1.
4. Betty and Veronica met Grunge in in Gen13 #13A.
5. Grunge met Spawn in Gen13 #13B.
6. Spawn and Rick Grimes both appeared in I Hate Image.
7. Rick Grimes and Herschel Greene appeared together in issues of The Walking Dead.

For a more immediate connection, albeit unofficial and merely implied, Mark Hammond in the premier episode of Z Nation referred to "an ex-cop with his group in an old prison around 20 clicks from here”.

zuckyd1 wrote:

1. 10K and Lt. Roberto Warren appeared together in episodes of Z Nation (an Asylum/Syfy production).
2. Roberta Warren (then only a sergeant) and Fin Shepard both appeared in Sharknado 3: Oh Hell No! (also from the Asylum and Syfy), in which a sharknado threatens Washington, D.C.
3. Betty Cooper and Veronica Lodge were caught in that same D.C. sharknado in Archie vs Sharknado #1.
4. Betty and Veronica met Grunge in in Gen13 #13A.
5. Grunge met Spawn in Gen13 #13B.
6. Spawn and Rick Grimes both appeared in I Hate Image.
7. Rick Grimes and Herschel Greene appeared together in issues of The Walking Dead.

That'll do it. You could have also used the character of Doc from Z Nation, as he appeared in Sharknado 5, or, bizarrely, writer George R.R. Martin, as he played himself in both Z Nation and Sharknado 3! And Sharknado is the event needed to get out to the wider Omniverse, as afaik none of the characters from either Z Nation nor Sharknado appear elsewhere, but as you noted the Archie gang had to deal with the Sharknado too. 

The Archie team's many crossovers make them nexus points for linking the Omniverse and crossing over to Image, but none that I am aware of that would be shorter than your route. And there Rick Grimes is the route into Walking Dead, as he is (afaik) the only character from that to officially appear in other titles.

zuckyd1 wrote:

For a more immediate connection, albeit unofficial and merely implied, Mark Hammond in the premier episode of Z Nation referred to "an ex-cop with his group in an old prison around 20 clicks from here”.

Yeah, I remembered that nod to the similar show that likely inspired Z Nation, but as you note, it's a nod and wink, not an explicit / official crossover. If it had been the challenge would have been way shorter!

Loki wrote:

zuckyd1 wrote:

Well it's more of a fork than a straight chain, but how about this?
1. Nightmare to Doctor Strange in Strange Tales #110
     2. Doctor Strange to Morpheus in San Diego Comic-Con Comics #3
          3. Morpheus to the Corinthian in issues of Sandman
     2. Doctor Strange to Ash Williams in Marvel Zombies/Army of Darkness #5
          3. Ash to Freddy Krueger in Freddy vs. Jason vs. Ash

Not bad. That'd be acceptable, but I wonder if Strange's appearance in SDCCC#3 counts as authorized? 

I thought we had decided the San Diego comics counted?

In that case:
1. Nightmare to Doctor Strange in Strange Tales #110
     2. Doctor Strange to Batman in DC vs. Marvel #1
          3. Batman to Morpheus in Green Arrow (2001) #9
               4. Morpheus to the Corinthian in issues of Sandman
     2. Doctor Strange to Ash Williams in Marvel Zombies/Army of Darkness #5
          3. Ash to Freddy Krueger in Freddy vs. Jason vs. Ash