Mimic was indeed a minion of Onslaught, they he did absolutely nothing. He was rescued by Onslaught and given control of his powers (off panel). He showed up at the end of a later issue and baled Blob out (who was also enhanced by Onslaught). And that was the last we saw him or Blob as minions of Onslaught. It was a wasted story, as Mimic has the potential of being a great character.
Oh, and Post is indeed dead. He was killed by Pryo during the Dream's End storyline. He was attempting to assassination Senator Kelly but was fried by Pryo, who then died of the Legacy Virus. Good issue.
And man... I LOVED Abyss in both AoA and 616. I'd love to see him get a bigger role somewhere, and as Nightcrawler's half-brother, I'd think Dark X-Men would be a good place for him. He doesn't have his powers, but they were technologically returned, so it can work.