F.Y.I. final update.
Something seriously wonky was going on with mine despite the flawless upgrade and a new empty database, but login issues went away. I assume the login issues for me was that ad-hoc conversion from MySQL to MariaDB. From the start, I mentioned that "mod_assignemnt" error and it was a big problem for me today. I didn't know it at first because I had no error messages about it anymore. After what I outline below, I realize moving the "mod_assignment" folder back in was a crucial error.
No course restore attempt was working properly; my 1st attempt was stuck over 9 hours. I couldn't delete after the frozen restore either because the restore was in limbo. I tried many permutations with many different sourced backup files as well as module partitioned backups. I had to manually remove the rows in the appropriate MariaDB tables directly to clear the course. I edited my "me.moodlecloud.com" version to remove the only "Assignment" in the entire course, then made a new backup of that. This new version restored perfectly. However, I could not add an "Assignment" to the restored course on my server.
While looking in my "*.com/admin/plugins.php" Plugins Overview I saw I had a disabled version of Assignment (2.2) still installed, so I uninstalled that. I removed all the PoodLL plugin's too because I don't really need them. I assumed Git would have pulled perfect code, but for some design reason Moodle changed the mod for Assignments to "mod_assign" from "mod_assignment". This allowed two competing programs to be available despite the older one being disabled.
Everything worked right after that, even the links for the question banks reappeared! I was immediately able to create the missing assignment. Moral of the story is somebody should be telling people with the 2.2 version of Assignment's to remove it before the upgrade to Moodle 3.6.