So while I have no real breakthroughs, here is some information about the engine in general.
Quest for Glory V seems to be a 2.5D engine (some 3D objects interacting essentially in circular rooms using static background images) rendering output in 15-bit RGB (while using floats internally and paletted assets). The engine logic is hardcoded, partly inside the executable, partly inside dynamically loaded room modules (in Windows or Mac native format). From what I’ve already seen, there are several global objects responsible for various parts of game engine, including the main engine class with around 160 callable methods; room class takes a pointer to it and provides around ten methods that can be invoked by the engine (and which in their turn may invoke engine methods for various actions). So it’s about a megabyte of engine code and over four megabytes of code in room modules. At least I don’t need to decompile them all at once.
The game data is organised in stand-alone files and SPK archives. Cutscenes are Cinepak in MOV (though IIRC my official pirate copy re-compressed them to use SVQ1 instead), audio is MS ADPCM in WAV. Overall there are fifteen resource types known (audio, lipsync data, text messages, panorama background and its palette, 3D models and its textures, GUI decals and so on). Most of the files are contained in SPK archives which are essentially slightly hacked ZIP archives with initial header replaces with custom index per resource type and each PK entry has numbers changed so it won’t be detected as a ZIP archive without header (at least there’s no compression employed as far as I can tell).
So overall I just need to discover how the main engine loop works, what are all those file formats and reconstruct the room logic. That should take a long time (supposing I don’t give up earlier). Either way I’m doing it mostly to pass time and to find out how far I can get REing something more complex than a codec.