As for the specks, Maybe there’s an automated tool to clean up film grain that would pick these up? Otherwise I would happily go in and clean them if it meant salvaging the end of the shot.
If it is possible to clean up some of the soot, I think that might be worth it. Right now, it’s the only thing standing out to me as betraying that the shot was sped up; it almost looks like digital noise at this speed.
But that said, it’s a very minor thing, and the rest of the shot looks phenomenal!