A lot to agree here.
The puzzle-nes of the Grym encounter is not really great. Doing somthing like that in a real campaign would be considered a dick move, since the normal run of the mill 4th level party would just be one thing encountering it: dead. The big problem here is: they expect us to examine the statblocks. But in reality that is heavy meta gaming. In a normal playthrough I'd like to not rely on it. Same goes for the lava effect that weakens it. It is just so very very gamey. And not in a good way, sadly. An encounter where there are just 1-2 "right" answers to it is not fun. Just make it a though construct and maybe a few additional minion constructs (so the action economy is not totally RIP against the enemies).
I had no issues with the merregon encounter, but maybe that was just luck. If they really have a fly speed or teleport or whatever: yes we need less of it.
Mimics were great imo.
The way to the forge should definitely be accessable to all kinds of parties. And lets all remember here that spell scrolls are not the solution to this:
a) they could be "wasted" before encountering the forge (we dont want to rely on meta knowledge)
b) scrolls can only be used if you have that spell on your spell list. So martial parties would be left out. And yes: getting just one PC over there and then use the teleport is really cheesy.
