Additionally, you mentioned running PEX up through the floor to the
fixture. I imagine they make a fitting to go from PEX to the threaded
fittings on the shut-off valves.

Yes, there is a stainless steel sleeve that you insert into the end of the
PEX, then you attach a regular compression fitting(shut off valve) and
tighten it down. Thats what you do if you dont want to put a copper stub on
the end.