RE: the Feelthere 175/195, Shift+E+2 are the only doors I've ever attempted to open with FSUIPC installed. In terms of other addons, I can say it's plausible, but hard to know for sure, particularly because once FSUIPC is removed, all runs fine.
Control assignments: I've actually, once removing FSUIPC, have been able to assign all the needed axis/controls via P3D4's native controls settings. There are a few VA applications (custom built ACARS) that I'm seeing run on FSUIPC as of now, so removing it renders some of those (one in particular) obsolete. Otherwise, I'd just remove FSUIPC all together if I didn't recognize any other ramifications of removing it.
RE: other key combinations, yes. I'm imagining the same thing the other person in that support thread is now happening to me, for reasons I can't determine. I'm sure enabling the logging will show that P3D4 is not registering the key combinations. Ironically, despite QW787's having a panel of its own to operate all of its exterior doors, the unknown ghost error of FSUIPC (allegedly) seems to be affecting the way that individual panel operates, also. Once the module is removed, the QW787 door panel operates all doors without a problem, unlike the video I showed with FSUIPC installed.
I've already reached out to QW787's support team, and they can't replicate the issue, of course. As of now, without completely uninstalling P3D4 and all components, which I'd hate to do, the solution is to remove FSUIPC completely. Only problem with that is then that renders some crucial applications (really, just ACARS), unusable.