From what I've read the barrel is supposed to recoil .030" before the bolt unlocks. I've watched some videos of the P90 firing, and I *think* I see that happening.
Given the relative mildness of the 5.7x28, I don't see where going straight blowback would be a problem. Recoil would theoretically be a bit sharper in blowback.
I've found some reloading information talking about swelling at the case head on fired brass, which is common enough on blowback actions.
The P90 magazine looks like any other magazine where it presents to the breech; the ramps and rollers that turn the cartridges are all in the magazine.
So far I haven't been able to tease any disassembly photos or diagrams out of Google or Bing. FN's web site has precious little information, either. Though none of it matters if you build your own innards from scratch.