I got it working 
UWP file format seems to be:
8 unknown bytes
png thumbnail (which ends with magic number 0x426082)
followed by the savegame data (ie: Savegamei.save)
I extracted everything after the png end marker, renamed it as Savegamei.save and it worked.
I think Windows 10 is ok: there are lots of improvements (the Linux subsystem works really well!).
What’s wrong is this UWP/Store stuff and the fact they do anything to push it forward (like allowing to package standard win32 apps and release them).
I personally don’t like the elephantness of W10, the lack of drivers to let older hardware to work, and the officiousness of Cortana. OK, I can permanently deactivate it by touching certain internal settings in the registry, but I don’t think that’s fair.
Every user must have an easy chance to deactivate it, if it’s too much irritating.
Anyway, we are going off-topic now.
Glad that your issue has been (partially) resolved!
A stand alone Linux system works even better. 
Don’t forget the need for activation (Ok, that’s also true for Win7) and the annoying in-game upgrade system.
Well done.
I registered for this forum just so I could thank you, @warpdesign , for figuring this out. Saved me some replay time when switching from the MS to the GOG version. Thanks a lot 