Rodd, I too have a Pearson with the same set-up as yours. I have replaced two sea cocks with new Perkos. The nuts backed off without a problem with me and, I'm sure with you also, there was no evidence that the studs were actually through bolted. I assumed that these were studs treaded into the backing plate, which is about 1/4" thick. Threading into the backing plate is an accepted method for securing the sea cock. Also, if you choose to use a pair of pliers on the threads, it is possible that you could get a tap on the stud afterward and straighten it out. I have used this method before. When you find out, I would appreciate you letting me know.
Thanks
Mike Meier
mbblithe/@gmail.com