t kills everyone, burns the cosmic commons to whatever extent necessary to eliminate any potential threat and then it goes about turning whatever is left into paperclips.

That's where Clippy might fail at viability-- unless it's the only maximizer around, that "kill everyone" strategy might catch the notice of entities capable of stopping it-- entities that wouldn't move against a friendlier AI.

A while ago, there was some discussion of AIs which cooperated by sharing permission to view source code. Did that discussion come to any conclusions?

Assuming it's possible to verify that the real source code is being seen, I don't think a paper clipper isn't going to get very far unless the other AIs also happen to be paper clippers.