Hey all, sorry for never replying to the suggestions you posted. I actually ended up fixing this problem on the same day I made this post.
The problem was that when opening up the project, if I also opened up my player blueprint the root component would be a camera and there was no spring arm or mesh - the properties of the camera root seemed to persist even after I recompiled the project to get the spring arm and mesh back.
However, this is how I fixed the problem: If I recompiled the project before I opened up my player blueprint, the mesh, spring arm, and camera would all be there in their correct places and the camera functioned as normal. Still, it’s really annoying to have to recompile the project every time I open it up…
At this point I’ve kind of given up trying to use C++ in Unreal for now and instead am focusing on getting stuff made quickly using blueprints. Though I’m still interested in learning what went wrong so that this fiasco can be avoided in the future.