I've found a few sync issues like this that I was able to solve in the following way:
1. Use your web browser to go to lightroom.adobe.com - make sure you're signed in with your Adobe ID.
2. Look through your photos and see if any have a completely black thumbnail.
3. Delete these photos in the web interface.
4. Go back to LR on the desktop and sync again. The sync should complete.
5. Open LR Mobile on your mobile device again, and let it sync if needed.
Repeat as necessary until all photos are properly synchronized.
Your advice sort of worked, but I'm still stuck with 1 photo syncing that just won't go away.
I did find 2 "completely black thumbnails" on lightroom.adobe.com, but not all 6. I decided to just remove a bunch of older synced photos and all synced videos. That helped too, but I still have the stubborn 1 photo that won't sync.
I've gone in and deleted the sync.lrdata file again, but still no luck. Mobile apps seem to be working fine and show "all up to date" in the sync status. It's only the desktop version that's giving me headaches.
I shut down Lightroom Mobile without any success uploading the photo. It would have taken seconds to upload with the a wire from iPhone to computer. I am deleting Lightroom Mobile from my iPhone. It is just a source of frustration.
At this stage I have similar sentiments. Sync from iPad to cloud and down to macbook seems impossible. I delete the file in the collection as suggested above (web interface) and the sync starts (lightning symbol) but then most of the pictures end up with ! mark i.e. sync failed. My trial is about to end an right now I can see nothing to support me paying money for this service.