The IIgs UI is not simplified at all, it had pretty much all the toolkits the Mac had in terms of user interface. It had all the graphics bits, the IPC bits, even networking, layered filesystem etc etc; it is often overlooked how far they had gone in term of featureset at the time of GS/OS 6.03 which was the last (official) release.

Is any part of GS/OS based on original MacOS code ported from 68000 to 65C816? Or is it a clean-sheet reimplementation?

If one in 1988 wanted to port a MacOS application to GS/OS, how difficult would it have been? What would have been the IIgs equivalent to, say, MPW?

I think the main problem was that the chunkier graphics necessitated by the lower resolution always made the IIgs graphics look like a primitive copy, so everyone assumed the rest of the stack underlying it was similarly primitive.