My LrC Collections and Lr Mobile Albums are empty. What did I do wrong?

Yesterday, for reasons related to Lr Mobile for iPad's limitations, I decided to flatten the folder structure of my Collections/Albums from Collection Set>Collection Set>Multiple Collections to Collection Set>Single Collection. So, for example, I went from Oregon (C Set)>Crater Lake (C Set)>2021 & 2022 & 2023—each of the latter being a separate collection—to Oregon (C set)>Crater Lake (combined Collection).

I accomplished this in LrC by creating an entirely new Collection for each of my top-level Collection Sets and dragging all the images from the multiple older Collections into the newly created combined Collection. Then I deleted the old Collection Set and the Collections that they contained. I was not always presented with a dialog box, but when given the option to keep the sync'd images from deleted Collections in All Synced Photographs, I chose "No", understanding (I believe correctly) that these images would remain in any other Collection to which they had been added. And this seemed to be what was happening. In LrC, the images in my new combined Collections remained there after deleting the old Collection Sets and Collections. So far, so good.

But on the Lr Mobile side, which in my experience should just be called "Lr Crippled", things were not going as well. The newly created Collections appeared as new Albums with 100% reliability, but over 25% of the time, they were not populated with images. Over 600 images failed to sync from LrC to Lr Mobile. (The ones I noticed were primarily if not exclusively TIFs or DNG files.) After researching the problem, one suggested solution was to permanently empty the Trash in Lr Mobile. There were nearly 900 images there. So I did this—and this is what blew up all my Collection/Albums. They are empty now in both LrC and Lr Mobile. Fourteen years of carefully curated image sets are empty. 

What I don't understand:

• Why were there were images in the Trash in the first place? I didn't put them there. I was deleting Collection Sets and Collections in LrC, but the same images were present in Collections that were not being deleted.

• Why would deleting images in the Trash would affect images in Collections and Albums that were *not* in the Trash? This makes no sense to me at all. In LrC, images can be in multiple collections. Deleting one Collection or deleting images out of one Collection does not delete them from every Collection. Yet this seems to be what happened when I emptied the Trash in Lr Mobile. What am I missing?

• Why don't some TIF and DNG files sync? This failure is ultimately what led to my bigger problem.

• If I open a backup LrC catalog, I presume the Collections won't be empty. How do I keep Lr Mobile from emptying them just like it did to my current catalog?

I realize this is a lot. Thanks in advance for any help.