We have to accept that Gorn biology has no tolerance at all for subfreezing temperatures. The Gorn young are functioning at an instinctual level, they would learn by lethal experience. Unlike the adult Gorn in 2 x 10, they would not have the protection of environmental suits.
As I would very much love for Hemmer to be somehow still alive, having survived the fall and a period of low temperature semi-hibernation, anything that brings Starfleet back for follow up would be welcome.
I’m going with assumption that the plan to lure the more developed Gorn outside was successful, but could not address any new ‘infections’ in survivors. Hemmer repeated the plan, but as an Aenar, evolved for extreme subzero temperatures, and more physically robust than a human, there’s no reason to believe he died, no matter that he was willing to sacrifice himself for his shipmates. (I’m willing to go with his using his abilities and some kind of sharp tool to stop his fall and find a crevice to hibernate in.)
I suppose “Gorn just die very quickly in the cold” is the simplest explanation, but still leaves a lot open.
I’d love for Hemmer to return, but feel like the sheer height of the fall makes it quite conclusive that he’d die. Maybe I’ll “warm” to the idea (see what I did there?) if the writers can make it work reasonably. It would be particularly fitting if Hemmer’s unique physiology was the thing that means he can survive, as his preference for the cold is the opposite of the Gorn, and he even says “Just like Andoria!” before he jumps jumps off. Maybe if Batel is able to be saved from her Gorn infection, the new medical knowledge could be used to save Hemmer if he is indeed semi-frozen/hibernating or something. It could make for some interesting further development of the Aenar race also.
The possibility of revisiting the specifics of this episode could be why they’ve left these loose ends that I’m questioning in the OP.