The compression numbers look OK, nothing to be concerned with.
You can install a 3/8 gate valve in the "bypass line" with little effort. You can use slip fittings or pipe to attach to the t-stat cover, either way works. Just do it the easiest way for operation.
If you crack the "water valve" on the thru-hull does water gush to the pump? Follow the line to see if there is a strainer in the line or the thru-hull may be plugged too if no flow.
Was there a line from the Tee fitting to the t-stat housing? Just want to confirm there is for a bypass valve.
A good idea as you got hot is to check under the exit fitting on the manifold. This is a spot that can collect debris (especially if there is no strainer on the intake side of the pump) and cause flow restrictions.
Dave Neptune