• Did I goof in getting a valve with ¾ connections? (I bought it because I liked the porcher trim).
No, but you would be better with the 1/2 ported valve, that PorcherRP540 is available in 1/2" and the trim should adapt, but I would say the 3/4" would work.
• Am I ok using ½ copper from the supply (approx 9 feet of pipe) and from thermo valve to shower head (3’) in this situation?
BTW, good luck with the remodel.