Well if they bothered to get a sworn affidavit it could be used as evidence but only on things he knows directly and then it is limited because as you say, they can’t cross examine him. That is the very reason witnesses are killed.
Yep on all your points. But the timing seems like they wouldn’t have an affidavit so soon after he caved. They’d want to take their time, interrogate him some more, then get it just right. We’ll find out soon enough.