I downloaded the free DLC that says it is to play with friends who also have the DLC characters, since it was the only option. It shows the heros and villians on team roster screen and I can add them to my team and run around the stark building, but I haven't tried playing with them in a combat mission yet. I'm guessing they removed the other hero and villians packs because they no longer wished to charge players for old content and now use the free one to give access to all DLC content. I'll post if I am able to get the Cyclops/Jean Grey achievement I missed before.
Also, I had purchased the combo pack years ago and have saved games with the DLC characters in my roster, but I don't think that has anything to do with it. I was unable to access these characters before downloading the free DLC.