Third time's a charm?

Using the parallel port tester for to activate a charge pump is completely useless, unless you have modified the Hal file to generate the correct frequency.
So if you are using the parallel port tester without modification trying to the trigger the charge pump is useless.
The parallel port tester generally relies on the user having a few LEDs and switches connected. Either via a BOB or simply connected directly to the port. Or you may choose to use a multimeter and a piece of wire.
So I would suggest you try and test the parallel port NOT by trying to trigger the charge pump. If the parallel is on board, undamaged, enabled in the bios and you are using the correct address it will work.

So this part is where I'm the most ignorant. Debian 10 is problematic on newer hardware. There are countless pages directed to this, and lots of people have the same exact issue that I'm seeing (blank screen after install and reboot). It's not easily solved. For that reason, Debian 11 is a much better choice. But that means that I have to use 2.9 (because 2.8 won't build on Debian 11). Is this likely to be less stable than 2.8?

One of the most confusing parts of linuxcnc is the dependency of versions on different linux distributions. I'm getting the sense that this is less of a concern (at least with RT_PREEMPT) and that almost any linux should work as long as the software builds. Does that seem accurate?

If you have the knowledge and persistence it will work on your favourite distro, you just need to install the required libraries.
This maybe from your distros package manager or building from source, which would be the case with Slackware.
2.9 is stable as far as I know, many are using it.

Did you try the non free version of Debian 10 ?
cdimage.debian.org/cdimage/unofficial/no...ee/amd64/iso-hybrid/
It does require installing the RT kernel and LinuxCNC, but should be easy
linuxcnc.org/docs/2.8/html/getting-start...th_preempt_rt_kernel
Or if using a parallel port:
linuxcnc.org/docs/2.8/html/getting-start...#cha:Installing-RTAI

I haven't tried that iso, but I've added the non-free firmware to the standard .iso. I get a completely clean boot (nothing called out by dmesg) but the blank screen after boot persisted. There is a known kernel bug that causes this, but it doesn't get fixed until a much newer kernel. I've installed Debian 11 and Linuxcnc 2.9 this morning. That all went really well, but the parallel port is not responding. Since you asked for details, I'll try to figure out how to tell which chipset it uses, but its the same exact board that was working, as recently as yesterday on the MSI (when it has 2.9 installed but not 2..

Pick one machine to work with, install Linuxcnc and get it to boot.
Supply the info asked for regarding the parallel port as requested earlier. You were supplied with a link on how to do that.
Reread the post regarding the use of the Parallel Port Tester.
Please be consistent, swapping and changing is not helping.
Please try to approach this in a rational manner. You are not helping yourself.