Is there any way to get XMP sidecar files out of LR Mobile, or its corresponding Collection on the desktop, so that I can delete RAW files from my device while retaining edits?
Will the following meet your need?
1. sync with LR desktop
2. Move the edited photos to another folder within the desktop
3. delete from your mobile collection
Once you do that you lose the image on the mobile device. Also, you still can't get the XMP file even when the image has been moved.
I think this is to do with DNGs - I don't think LR will generate XMPs fro DNG files at all, so copying the DNGs from the sync'd mobile catalog, then deleting them from the device, is the answer.