We need a way to force gsx to use an afcad
There's no need to, since GSX respect the simulator priority so, if GSX cannot find an AFCAD, it means you DO have a conflict or a problem you might want to check.
If there's no conflict, the reason might be related to a file rename.
as explained many times on the forum, in order to speed up the cache regeneration time and the simulator startup time, GSX doesn't check each and every file you have (there might be dozen of thousands), but only the last modification date of the folder the file is located. This means, if the file is renamed, GSX cannot always tell the cache needs to be rebuilt, since the folder last modification date might not change, depending how the scenery has been originally installed. That's why there's a manual cache rebuild option.
Even if its a partial afcad for instance ray smith afcads updated for p3dv4 no longer work and they used to work.
Partial AFCAD were never supported by GSX so, the only possible reason they used to work but they aren't now, is they weren't partial before, and are now been changed to be partial. Assuming, of course, the issue is not the one explained above.