I went directly to the mine owner, Emerson, then the Amnish Soldiers, but no mine entrance transition point spawned. It was the same with every Amnish Soldier: they give the "I wish you guys luck in there. Whatever's been causing all the trouble isn't something I'd wanna run into," and then the soldiers on both sides of the central Amnish Soldier vanish, but that's it- no way in. I also tried talking to the soldiers prior to talking to Emerson- no difference.
It seems the missing element was that I hadn't been to Nashkell yet. Having already gone through the above steps, sImply visiting Nashkell to get the Chap 2 narration cutscene (still not speaking to Ghastkill) and returning revealed the mine entrance. After testing it in vanilla BG1 it seems the same thing occurs, so at least it's not an inconsistency. It's inexplicable that someone can't gain entrance to the mine simply due because they haven't set foot in Nashkel yet...
One of these:
1. Remove the Chap 2 requisite for entering the mines (not so good an idea since concluding the mines brings Chap 3)
2. Add dialogue to Emerson so that he tells you to go to Nashkell first (which is a clunky workaround since, again, there's no logical reason for the player to set foot in Nashkell before trying the mines, nor would there be anything about simply passing through Nashkell that should impress either Emerson or the Amnish Soldiers)
3. Move the Chap 2 narration cutscene to arrival at AR5400 (Nashkel Mines) rather than AR4800 (Nashkell) itself. Note that the narration content says nothing of Nashkel itself and instead focuses on the iron shortage and "trouble in the mines," so thematically and