After days of frustration I started this as a plea for help but continued working on the problem as I composed. Shortly thereafter came upon a solution.
Since I'd already written most of the plea I thought I'd post it anyway in hopes of maybe helping someone else. My apologies if this is already posted somewhere else. I've done so much screen time that my eyes still can't see straight.
So, . . .
new build:
MSI B550 GAMING PLUS
AMD Ryzen 5 2600
XFX AMD Radeon RX 570 RS
Fresh install of MATE 20.04.1 on one drive. No problems.
Fresh install of Windows 10 on another drive. No problems.
Machine ran flawlessly on either OS for a few weeks.
The MATE install was meant to be temporary until I got my NVMe drive which was to be my primary boot drive.
Got it. Installed it. Went to install MATE, and that's when the problems began.
The install USB is recognized and starts to boot, there's a flash of about 10 lines of 'do_IRQ: 1.55 No irq handler for vector' but the process immediately goes on to checking files, reports no problems, the desktop displays and that's where it ends, hangs? The mouse works, or at least the cursor moves about the desktop but it's an empty desktop, well, except for the MATE background.
I will cut, and not go into all the things I checked and tried.
What I eventually ran across was someone positing that there were two unrelated problems going on. The error messages were insignificant and the stall was actually a video card problem.
Stop the presses. Forget the fixes. Made sense to me.
I pulled the video card. Replaced it with an old cheap($30) card I had laying around. Booted Mate and installed without a hitch.
Replaced the video card and the system has been running flawlessly since. Such is life.
Of all the times I've installed or re-installed Ubuntu or MATE I've run into a serious problem only twice and they were different and never re-experienced. And now I've had a third.
I'm guessing that in the weeks that the system ran something changed to cause the problem. So perhaps my situation would be somewhat uncommon. And it is the case that in all my searching and researching of 'do_IRQ: 1.55 No irq handler for vector' problems there were only a couple that seemed to be associated with an OS installation.
So that's it for what it's worth. Hope it's helpful for somebody.