Not able to load custom mods from Content Editor

If I launch Content Editor and create a tuning pack, I can’t run Build and Launch: It will time out and eventually say “Unable to launch game” or similar error. I believe this is because I have the game installed into a custom directory rather than the default. If I build and then separately launch the game manually, it will not pick up the new mod if I go to Mod Manager and look at My Mods. There is an sga file in the mods\extension folder corresponding to my mod.

This used to work as recently as April. Any idea what might be going on? How do I fix this? Content Editor is version