So here was my final solution. It is, in a nutshell, creating a new user account, copying over all the user-specific files and data from old to new via the Shared folder, cleaning up preferences and syncing with iCloud, and, when 100% sure of success, deleting the old glitchy user. It was tedious but I can confirm it 100% works. Everything is done at the user level: no need to mess with any other folders on same level or higher up than the "users" folder. Below is a much more detailed description of what I did.
1) Begin by, from your existing user account, going to the Apple menu, System Preferences, Users and Groups, and create a new user (make an admin). It will walk you through a brief set up process similar to having bought the Mac for the first time. Supply your Apple ID and pw as requested, choose services you want or don't want, and complete the new user set-up. You can then login into and test drive the as-of-now-empty new user account for the issues the old user has been having.
2) Still from within your old user account, open a finder window and navigate to your home folder, within which will be the Desktop, Documents, Downloads, Pictures, Movies, Music, folders and possibly others. If you haven't enabled the "path bar" that shows at the bottom of the finder window you can do so in the view menu. It will make the navigation required for the remaining steps a lot easier on you.
3) Copy everything inside the old user's home folder into the Shared folder.
a) If you have Dropbox (or similar service), that service's folder may also show in your home folder, but you will not want to do anything with that one or its contents.
b) There may also be an applications folder that is user-specific, not to be confused with the main applications folder that all users can access. If you want those apps to remain user specific you'll have to do same with it as all the other folders. Or you can move those apps to the primary applications folder, where they'll be accessible to any user including a guest.
c) Do NOT copy the main folders themselves, as a whole, from the old user's home folder (ex: do not select and copy the Documents folder itself). Doing this will also copy the same permissions as the original and result in you not being able to access it from the new user's account.
d) INSTEAD, open the main folder (again, for example, the Documents folder) and copy all its CONTENTS (including sub folders) into a new "DocumentsTemp" folder you create inside the shared folder. Same goes for all the others.
4) Once everything you want to migrate is in the shared folder (preferably clearly organized in new folders as suggested above), switch to the new user account via the login screen, and sign in.
5) Navigate to the Shared folder, where you should see all the files copied into it from your old user account. Now, copy everything from those folders into the appropriate folders in the new user's home folder. Again, just like in 3(c-d) above, copy CONTENTS of the temp folders in Shared into the already existing equivalent folders inside the new user's home folder. Ex: open "DocumentsTemp" in Shared, select all files and subfolders inside it (cmnd+A), copy, then open "Documents" in new user's home folder and paste inside it.
6) Repeat for all data concerned—anything and everything you wish to migrate from old user to new. Yes, everything done up to this point may take a while, as you wait for files to be pasted into their new homes.
7) When this is all done (or even while it's still copying/pasting in other folders) you can test drive the new user. There should be no issues with full screen apps, multiple desktop spaces, etc. Customize prefs to be just like what you were used to before.
8) Go through as needed to make sure email, contacts, calendar, notes, Dropbox or Google Drive accounts, etc. have been re-established. All their data should be pulled down from iCloud and restored based on your Apple ID or other credentials. You may find the accounts are already waiting for you in the new user and just need activated and signed into.
9) When you are satisfied the migration has been successful and complete, and that things are running as they should under the new user account, you can go to System Preferences, Users and Groups from within the new user and delete your old one. Keeping the old user won't hurt anything except available hard drive space, so there's no rush to remove the old user until you are 100% sure you haven't missed anything.
10) You will be reestablishing preferences, desktop wallpaper pic, email signatures, etc. for a while as you run into them, but the critical things like keychain data and cloud-based data such as contacts will have already been restored.
If anyone uses time machine, you'd probably find it better to treat it like migrating to a new Mac. Link to how to do that in one of my previous posts in this discussion.
Using the Shared folder as a bridge between old and new user account is tedious. But it works! I am now completely 100% glitch free, no loss of files or function. And I didn't have to wait for Apple....
