Welcoming someone "with open arms" means you genuinely welcome them. You want them to be there. No one wanted Sinister to be there. They all hated him and were counting down the days until they didn't need him anymore. As for getting a hold of his archives without his cooperation, apparently that wasn't possible. It's literally in the story that they were taking DNA samples from every mutant resurrected to build up their own database as quickly as possible. That's how much they didn't want him there.
I'm not arguing that it didn't blow up in their faces, because it very clearly did. But there's no need to pretend the story was something it wasn't. Sinister was only there because they needed him for their resurrection process, not because they thought he deserved redemption.