While I can't put my finger on what exactly did the trick, I'll list the 3 consecutive? steps I took.

First off, I flashed my UEFI to v4.50.?

Secondly, I cloned my OS drive from the Plextor M6e in it's m.2->PCIe adapter to my Crucial MX300 in the native m.2 slot. Rebooted and set the boot order to load from the MX300 (and removed the M6e from the boot order completely) to test it. Succes, so I powered down and removed the Plextor from the system.

And the third step was to reinstall the chipset-drivers for good measure (+reboot after that) and Google how to boot into safe mode under Windows 10 (last time I've ever had to use safe mode was under W7, so a while back...). In safe mode, I ran DDU, which removed the Microsoft default display drivers and blocked their reinstallation on reboot (a nifty trick, that, and one of the major factors in the solution of my issue, I feel), and on reboot, made a system-restore-point for good measure and installed the AMD display-drivers again.

Needless to say, certain muscles in my body clenched during the installation, but the display came back on and I was so relieved :D

On a side note: is it normal to have an extreme Vcore like 1.45v by default? I can't remember what the 1600X in my gaming system rocked by default, but I know it can handle 3.8GHz at 1.325v.

So I decided to undervolt my 2200G, too, using the AMD CBS by -0.100mv.

If I had more fine grained control instead of steps of 0.050mv at a time, I'm confident I could lower it even more.?

I used CBS, because the Asrock option in the UEFI didn't seem to do a thing - I set my target max-clocks and Vcore (3000 and 1.325ish) and nothing changed.

Oh well, all in all, I'm very happy with the board and chip -- a major up-grade from my Kabini based Athlon 5350 I was using before :D

I would try DDU under safe mode as you mentioned then install the driver version AMD recommends for the APU. From what I have seen with reviews on the R3 and R5 APUs this is not an uncommon issue. Many reviewers complained about how difficult it is to get these APUs working properly. 

As for the M.2 slot issue, it may have something to do with Ryzen APUs not having as many PCIe lanes as regular Ryzen CPUs, the M.2 slots are usually designed to utilize spare PCIe lanes so as not to take away from the x16 slots. If the APU has less PCIe lanes then it is quite likely that the M.2 slot is trying to use lanes the CPU does not possess.