I would doubt if the OS3.1 would make the difference, but if you have a spare HDD it might be a simple method of staging the install to see what affects what as you go through the process.
I agree with your comments about OS4 Classic, and hope that as Hyperion have the rights to OS4 they do improve the OS4 Classic release to where it should have been ... stable and working !!
As you say, allowing the DMA 'hack' that Elbox brought out for the A1200 Mediator might cause issues, but why not allow it, after all it only applies to the A1200, as the A4000 Mediator allows DMA I believe. Also, who is it going to get hurt? Classic A1200 OS4 sales aren't really rocketing skyward out-of-control as the software was never stable unless you were lucky enough to own a B-Vision, or like using AGA with the constant out of CHIP memory warnings situation, and the limitation of 256 colours, and a prohibitively slow Workbench.
I have a Mediator TX, Voodoo 3 3000, Radeon 9200, RTL8029 Ethernet and numerous other Mediator based cards, but after my initial attempt to install Classic OS4 I gave up as all the limitations (incompatibility) just made it more trouble than it was worth, what with no Spider USB, no Soundblaster sound, it really was a step back.
I am due to give it another try (Installing Classic OS4) and if I get any further along with it on my next attempt I'll be sure to let you know.
Well you should ask Elbox to write proper OS4 Drivers. They haven't yet, to date. OS4 is a completely different OS with different kernel and memory management system, so it's no wonder hardware drivers written for OS 3.x aren't working.
OS4 classic supports both Prometheus and Mediator bus boards "out of the box" using the Voodoo 3 graphics card. It also supports the Cybervision/Blizzardvision graphics cards.
Radeon is not supported (there is no driver). The ATIRadeon.chip that is included with OS4 requires the execution of the Radeon's x86 BIOS. Unfortunately on classic there is no x86 emulator to execute the Radeon's BIOS so the chip fails to init and thus infinite loops. You can try to use Elbox's OS 3.x driver, as it pokes the card's BIOS directly (that is why only a handful or two of specific Radeon cards are supported).
If you follow ACube's compatilbity list for OS4 classic I think you will find it works quite well. I have three setups going and the Mediator and Prometheus support is good with Voodoo 3 and Realtek 8029 PCI Nic.
