I have thought about how to do atomic payments to multiple parties at once over Lightning so much... thanks for laying out the problem space like this and theorizing a potential way Bolt12 could help do it. It's a really interesting idea.

What I struggle with when it comes to all these approaches is -- and this is coming from the perspective that the reason we need atomic split payments is so there is no custodial element when trying to pay multiple recipients simultaneously -- every participant still needs a stable, funded Lightning node for the payment to work at all. I think right now it is still way too difficult for most people to run their own node. And until that hurdle is cleared, non-custodial split payments don't gain most end users very much if those payments are ending up on a node they don't control anyway.

I'll have to read more into the blinded paths pattern in Bolt12. If anything it could serve as a good enough solution in the near future while nodes become easier to run over the longer term.