Thanks for the suggestions!
The display is just the iMac built in 27" LCD display (no additional monitors connected):
Resolution: 2048 x 1152 (note I've picked this and also picked lower resolutions when choosing the window size)
Pixel Depth: 32-Bit Color (ARGB8888)
Graphics chip: NVIDIA GeForce GTX 675MX 1024 MB
I'm not using Steam - both times, I've downloaded from the Spiderweb Software website.
With no other applications running and after freshly restarting, I made it a smidge further (talked to the prince, tried to open the chest *flicker, flicker*).
Restart again, having saved this time before opening the door north out of the hallway and loading saved game - froze on opening the door to the throne room and having the text pop up.
It seems like opening things is my downfall.
I have Avadon 1 and 2 on this computer and Avernum 1's demo; they don't show the same issues after some testing.