ST, how plausible is this scenario?: miners signal starting next week but then don’t produce compliant blocks when it activates in September, leading to a split in Sep, not next weekend.

I have to think about the Billy's +0.02% of the network -- I see your logic but am still wrapping my head around whether the benefit to Billy starts immediately or if it more substantively starts in the subsequent epoch (assuming the 2% is gone indefinitely). More generally, at one point I was thinking any remaining mystery would be over after next weekend, but then I thought that if one of the big 5 does start signaling, perhaps others immediately join in on signalling too simply to buy time/defer the split for another four weeks.