In the western section of Rapax Rift is a sealed door (needs a specific item) leading to a passage that runs parallel to the one nearby that opens into a bath chamber. There's a small lava-filled crevice that breaches the wall between these two passages.

I don't remember if that's where the Staff of Ash is (which I need elsewhere), or if it can't be entered until later. I vaguely recall being able to slip through the lava-filled crevice back when I first played the game, but I'm unable to do it this time, and no key that I have (Sanctum Key and the Flamequencher Wand) open the door. What am I forgetting here? (As far as I can tell, I've cleared out everything I can otherwise reach.)

Ah, there's the thing I needed to find...

I don't remember whether I had the same problem the first time around (10 or so years will do that to you...)

FYI the lava is one-way... the key is with the staff.

As regards the small lava pool between the two passages, you need to bring down the support pillar in the hallway you can access, and the resulting rubble will create a makeshift bridge across the lava. Just click the hand icon on the metal beam. It might be tricky to spot, but it's there.