Can I upload videos made with other people's models and motions but with my camera or does it have to be 100% original?
yeah, go for it, everyone does that here
You can upload your video work.
Improving a motion,cam,audio. Fixing Problems with Models and using and uploading is OK. Thats what shared MMD Items are for.
If you use someone else's model (or motion, or camera, or stage, ...) I would appreciate if you provide information about that in your comments. Maybe I would like to also use that model.