Durlag's Tower with 2.6

[Taylan]

I'm playing on the 2.6 beta and I'm at the torture chamber fight, where the enemies spawn behind you in the torture chamber as soon as you walk into the next room where the book saying "face my demons" lies on the ground.

1. It seems that the enemies that spawn are semi-random. I seem to get a selection of two from the following: Durlag Trollkiller, Islanne, Fuernebol, and Kiel the Legion Killer. Is this intended?

2. Sometimes one of them will spawn directly as a Greater Doppelgänger and not as the more powerful character they're supposed to be. This is almost certainly a bug as it makes them much less challenging. Not sure if 2.6 is to blame.

3. I couldn't quite figure the rules of when they turn into a Greater Doppelgänger. It seems that they usually turn when hurt, but sometimes it takes much longer. I was once able to kill one without it turning, dropping a Full-Plate Armor! Another time I had one turn back into the imitated character instead of dying as a Greater Doppelgänger. What's the actually intended behavior here?