The video posted by u/hansenq showed what flows were like, with traffic from the east sequenced as far out as Salt Lake City. It probably would’ve needed coordination with Oakland, LA, and Salt Lake centers
That doesn't add up with far-out sequencing.
As far as I understand all they needed to do was to move one plane down from the parallel pair and increase lateral spacing a tiny bit, all the way back while the plane in question was somewhere over Utah.
No, that's also not how it should work. Airports have a certain capacity, that's why you use slot allocations at crowded ones. You can say that day was so botched that they blew the one plane in fav of all the others, but that is not how it should work, telling a plane with an allocated slot you cannot even have a realistic estimate when we will fit you in.
You’re not wrong, but don’t forget that they were three hours late. This is really why airports operate on a first–come first–served basis; someone is always late.