Tried using Final Alert 2 ( the unofficial version ) on Windows 10 x64 Home.

On attempting to open a map, got a thread access error, crashing the editor.

Should I assume that it is incompatible with Win 10?😞

I've been down that compatibility rabbit hole before, and it can be a bit of trial and error. As I mentioned, running it as an administrator and experimenting with compatibility modes can often do wonders. Sometimes, it's just a matter of finding that sweet spot where the two can work together. But if you're still hitting a brick wall, don't throw in the towel just yet. The Window 10 key  Reddit sub is a fantastic resource. I've seen some real tech wizards over there, and they might have some creative solutions up their sleeves.