Can I make some comments as I see your post explanations?-
1) "I transferred everthing from old PC to new". How? Ideally you should have done a fresh install of the apps from the CC Desktop App (not transferred in any other way). Although your re-installs would have corrected this.
2) I see all your catalog files are being synced (with Dropbox?) as there are the status symbols evident- Stop syncing catalogs with any Cloud feature (many reports of incompatibilities).
3) "imported all of my many images." Yes that was the wrong thing to do (as mentioned by @dj_paige )
4) This appears to be your latest working catalog- are you [Double-clicking] this .LRCAT file to open the catalog? (STOP the syncing first!)

5) Backups are zipped (compressed) versions of your catalog that are saved in 'Date' folders.

To use a backup catalog as a test you need to extract the .LRCAT file from the ZIP folder, move the .LRCAT file to a new location (eg. Desktop), [Double-Click] the .LRCAT file to open the catalog (that was in the backup).
I hope my comments are helpful.