To what extent to I need to be concerned about PCB punch-through when dealing with AC mains?
IIRC.  0.4mm for a single layer of insulation.  Can be thinner for two layers of insulation.  UL standard assumes that there can be a defect (void) in the PCB, which can be a hole for the arc to go through.  If there are two layers, then it's less likely that the defects in both of them will be in the same place.

P.S. I've looked this up last year, when I was considering a planar transformer with UL isolation.  Will try to find my notes.