i'll plough on while i have some energy.
the first shot is from my linux mint 19.2 'tina' MATE, which was what you had before you upgraded.
the second is from my linux mint 19.3 'tricia' MATE, which you upgraded to.
by right-clicking an empty space on the panel, the context-sensitive menu revealed includes an option to reset the panel. if sue has not tried this, she can click to reset and see what happens, if anything.
other readers should read on.
in most cases, applying the panel reset button will restore factory defaults, and we'll be left with
- on the left, menu button, show desktop, file manager, terminal, and firefox
- on the right, the default system tray
if i pressed to reset with either of those linux mints above, the 11 - 13 icons for my apps and launchers would disappear. the apps would still be onboard, but i would have to re-populate the panel with the quick launch icons.
subject to how sue responds, the next option we try might involve introducing a different desktop environment.