[Renate]

Just to confirm:
You used edl.exe to get your boot files, long and short.
Things were working fine still after the last time you touched edl.exe
You could still get to normal Android.
After juggling problems with drivers and fastboot is when you stopped being able to get to Android (and ADB).

There is the other set of slots, but we don't know what's in there and it would be risky to just switch over.
Has this device already had one normal update to it?

You can do a "fastbogus getvar all" and compare it to the first one you got.

You might be able to "fastbogus boot long.img". Try it.

I finally got your Magisk'ed file.
It looks normal, but I'm not sure if the DTB is missing and it's using dtbo or what.
I'll have to look at the long.img tomorrow when my internet speed comes back up.
How about uploading it somewhere and PM me the link, please?
As long as you hold onto those files and we still have fastboot we haven't dug too deep a hole.

That's it for me for the night. My apologies.