Thankyou for that input, you’ve explained the gaps I was struggling with.
The big mistake I made was taking “to protect their civilian passengers” to mean some other civilians we didn’t see, when in fact it meant the remaining two castaways (Buckley and Oriana). I didn’t give enough weight to the facts that the area is prone to sporadic heavy snow storms, and that there was no time for a thorough initial search by the Enterprise away team. It is possible the Peregrine crew deliberatly launched their plan just before a big storm was incoming, to make sure the Gorn would be trapped in it, which of course makes sense that the crew themselves would all die from Gorn attacks or from the storm (as supported by M’Benga’s assessment of the bodies outside).
I’m still a bit hung up on the plasma grenade not killing the gestating Gorn, but I suppose it is reasonable. I think the Peregrine captain’s log even mentions “but he didn’t succeed” (in killing the Gorn inside himself) or something. If gestating Gorn can survive later stages without being “attached” to a living host, this may also give some clues into possible twists with the Batel situation.
I will rewatch with all of this in mind. Thankyou for untangling my brain-knot!