Hot water never reaches 2nd floor bath
SInce you know hot water is making it to the 2nd floor, there are pretty much two possibilties.  One, as TPE suggested, is that you have a thermostatic shower valve that's not working -- or even a mixer valve that isn't opening on the hot-water side.  The other is that the shower is on a separate hot-water feed line from the sinks, and that this line is kinked, or has a cut-off valve somewhere that is shut or jammed (or is just letting a trickle of hot water through.

I fear that without being on-site none of us can tell what your plumbing "map" is.  My usual advice is to ask your friends who they have used and recommend for plumbing work, and explain in the same detail as you did here what's wrong.