Good morning Jeff,
Your solution of running FSX from the "C" but linking the scenery files from "P" is sound. I believe Francois D offered that advice to a similar issue in "another place".
I had some issues with my FSX install a couple of months ago. FSX would start, the flight would start, then a straight CDT. The advice I got from another forum was to clear out the flight plan file in the My Docs FSX folder. It worked. I have also cleared out a lot of add-ons; scenery, aircraft, apps, etc., so that FSX is now pretty much specific to the flying I am now doing. Currently thats PNW, PFJ and Tongass. Low and slow, no heavy metal, etc. The only other add-on's I have now are My Traffic for AI and Flight Keeper for flight logs, etc. All the aircraft variants I don't use are "//" in the aircraft config file so they are disabled but still available if required.
In the scenery library all those continents I don't go to are unchecked, e.g., Asia, Africa, Eastern Europe, etc.
All of that, together with regular registry scans, cleanouts and de-frags means that FSX loads and runs much more efficiantly. Everything is still there, apart from the flights, but it's only disabled - and it's all backed up. That's a good thing 'cos the computer's in the repair shop - new Motherboard!!
Happy flying... soon.
PS: the issue with the flights was a "saved flight" for an add-on aircraft variant that HAD been uninstalled