I think I may have solved the *.jpg.thumbnail and "forever" saving mode issues. These are the steps I had taken. Please note that this is not a refined method, as in some steps may be unnecessary, but I don't have the time to figure out which are the critical steps.
Hardware: Lumia 930 On the Fast Ring (14977.1000). I was on the Slow Ring when this happened, so I thought going on the Fast Ring.
Briefly: The folders in "Pictures" folder is the culprit.
Step1: Connect your phone to computer and back up everything in the "Pictures" folder.
Step2: Delete everything in the Camera Roll folder. (If you had accidentally deleted this folder, it is ok, it will be recreated when you restart).
Step3: I only have two other folders untouched..."Saved Pictures" and "WhatsApp".
Step4: Delete all other folders in your "Pictures" folder except Camera Roll, Saved Pictures and WhatsApp (If you use this app).
Step5: File Settings....Linked Duplicates = OFF. Camera auto upload = ON. Include videos = ON. File Sources, show my cloud-only content from One Drive = OFF. (not sure whether these are important...you can skip this step and just go to the next one)
Step6: Restart your phone.
I had fixed it since yesterday and initially, you may still see the .thumbnail under file info for the pic. But the actual full res pic is in the folder if you check with your File App. I did another restart and things worked out.
Shooting with HDR ON, tested OK. Shooting videos, tested OK. Shooting multiple pictures (JPEG 16MP, 16:9) - shot 13 pictures before the word saving came on...but it was only for a few seconds before it went away and I can resume shooting.
Other unsuccessful things I have tried:
- soft reset
- installing Lumia Camera Beta
- clearing my Camera Roll Folder and restart (this only worked to remove the "Savings" but not resolve the thumbnail issue).
Hope these helps. Merry Christmas!