Changed around some of the variables, but none seem to have effect.
Some changes I made:
In ENVARC:Mediator:
Warp3D=No
VoodooMem=12
VoodooInt=No
In ENVARC:PowerPC:
force=1
In ENVARC:Picasso96:
ShowModes=All
I do also see in threads like
this one that ENVARC:Warp3D should exist as well, but I can't find this drawer in my system.
The Warp3D that can be downloaded from Elbox seem to be the exact same version as on the Mediator CD. I installed this to see if it made any difference. The file versions are the same, and it copied over the Amiga 4000 files, just as the installer on the CD. I manually removed these and copied over the Amiga 1200 files. In the end, this made no difference to running the 68K demos.
With all these changes I at least haven't made anything worse. PPC demos run fine, but 68K demos do not.