Since nobody else has commented on my mess or your many helpful replies saivinob, I'm now in the process of backing up what I hope is the correct data partition. Then I'm going to run Timeshift on what still seems to be the root partition (glad I partitioned the drive!). Then I'll try the idea of reversing the CLI commands that I foolishly used to hose up a nice MATE environment.
If that works, I'll try changing the username via MATE's cool little System Settings / Users and Groups / Users Settings GUI. While waiting to see if you or anyone else offered further input, I realized the MacBook used for initial setup that I'd copied to this active MacBook and then added many further tweaks plus my data, still has MATE running fine. So today I checked and easily found that GUI username change option. It seems likely that will work fine, but can post a followup regardless.