I didn't read through everything, but I recently upgraded my Macbook Pro 15" (2015, no dGPU) from High Sierra to Mojave (latest Developer Beta 6). I had several glitches in High Sierra and would have to reinstall anyway, therefore I gave Mojave a test. Everything works just fine, beside the little detail that iTunes completely scrambled my audio books library. There was only one or two audiobooks from the Apple store, the rest was from Audible or from scanned in CDs.
When I select an individual author, like e.g. Suzanne Collins, I just see the three volumes read in from CDs. The same holds true for all other authors, but when I select "all authors", I see most audiobooks multiple times (10 to sometimes more than 20 times).
Music and movies seem all right. The audiobook files in the iTunes library have not multiplied at all, but all files from the three volumes of Suzanne Collins (as an example) are lumped together in one folder. I don't know how this was before upgrading to Mojave because I never looked there before.
The file system used for High Sierra was HPFS+, now it is APFS. I don't know if this could be the root cause of this mess.