Ok, I think we may both be getting a little confused. Or at least I am lol. So let me clarify everything a final time just to be sure.
1 - The issue we're having is when opening FSUIPC from the P3D v5.3 Add-Ons menu. It appears black, meaning only the actual ~10cm x 20cm box or window of the FSUIPC control assignment interface but it's all blacked out. (and the background behind it is too, but that's normal if I remember correctly.) If we grab it (the box only) and move it to another monitor, it then appears.
2 - When installing FSUIPC, and also summarily re-installing it, it appears that it is indeed installing to C / documents / Prepar3D v5 Add-ons as that is apparently where FSUIPC wants to install by default. (and you said it would do so by default due to any older installs so that makes sense if so).
Regardless, I didn't see any information during install (i.e.; in the installer itself that it should be located anywhere else but it sounds like, yes, you are telling me to just make sure it installs to C:FSUIPC instead. (and I assume you mean a folder of FSUIPC6 not 7 as 7 is for MSFS2020, no?)
And if I may, ...my main tip here would be that you guys might want to add a note/words of install location to the installer itself as I assume most people re-run the installer over a previous one as that has always been the normal operation for FSUIPC.. as it uninstalls the old one itself for you and re-installs the new. And therefore why would anyone think there's a need to install it anywhere other than where it defaults to. I hope you see the point here.
3 - Lastly, also in C / Documents is an FSUIPC6 folder but it appears to only contain ~15 PDFs for the User Manual, History, Lua stuff, Offset Mapping, etc. I assume it will maybe reinstall that data there again even if I point the new main install to just C /FSUIPC6 correct?
4 - My last question is with the .INI and backing it up to save control assignments. Can I copy/paste the entire old file to fully overwrite the new one or do I need to carefully cut and splice in only the old control assignment data I wish to keep into the new .INI ?
Thank you for all your help and prompt attention this week. It's been very helpful.