About attempts to bring back non-metal-acceleration , I guess we should consider that Catalina uses all the binaries with 64 bit coding, while instead all the frameworks/privateframeworks binaries taken from a Mojave installation will contain both the (FAT archive) architecture x86 and x86_64 , while the "mach 64 bit only" can be found on frameworks taken from a Mojave's BaseSystem.dmg , so I'd consider to use those.
I've also tried to replace only CoreDisplay.framework from 10.14.3 and the interesting thing is that from a verbose booting it doesn't crash and doesn't produce any pid crashes, but right after the stage2 apple logo (that will be showed for a moment during the verbose mode) it will loop on IOConsoleUsers verbose lines.
Of course in drawing the LoginUI or loginwindow are involved other very tricky frameworks, one for all the SkyLight privateframework.
Maybe who knows even in some OpenGL.framework for example could be suffice to replace only its binary exec leaving the Libraries or other subfolders untouched from Catalina. But unfortunately the elements to consider are many.
Maybe this time also the OpenCL.framework is required to support an OpenGL.framework downgrade.
The semi-good thing is that frameworks (differently from kexts) can be replaced almost on-the-fly from single user mode, and their behavior can be evaluate directly from verbose booting and if some pid will crashes, then almost surely that chosen framework downgrade isn't good, attempt the next.
But I have one more observation to add, I guess that the less tricky GPU to test if on Catalina exists still an OpenGL fallback is the IntelHD3000 but the one used as singleGPU on MacbookAir 2011 or on MacBookPro 13" 2011, since these GPUs were the first that worked will full QE/CI on Mojave 10.14.0 beta1 almost OOB, and their kexts were taken from a Sierra installation, so just think, Sierra GPU kext brought the first non-metal-opengl-acceleration in Mojave, it was incredible, and this motivated to bring acceleration to other non-metal GPUs, since it was proved that OpenGL fallback was possible on Mojave.