Crashing is a pain.
I had problems going in the other direction. After exploring the cave I could escape via the portal but no matter which other portal I went to, the game would crash as soon as I tried to save. I was only able to fix it by reloading an older save and re-doing the cave. So, unfortunately, I never found exactly which bit was buggy. Sorry I can't be of more use.
But I was then able to successfully return to the cave later.
Parts of the cave do seem a bit erratic. For instance, I'm not sure what was supposed to happen with Billeh Gahr, but it didn't seem to match his threats.
Where do/did you find the button to get the key? I never found either a button or a key, but I managed to get in by exploiting a loophole.
There's a small room to the left of the Black Ominous Gate. If you walk into the right hand corner you can squeeze through a gap at the end of the wall and end up standing on a ledge at the top of the stairs inside the gate. You can't walk directly onto the steps but if you toss a teleport pyramid to the right spot at the top of the steps - bingo, you're in. It works in reverse to get out too.
