Hello Tim, [snip]
Great news and a fascinating read.
I have a production PC running LMDE that I will need to upgrade at some point, a process that will include the same jump from grub-legacy to grub-pc as you've just fought with.
Might leave it until the weekend!
There are now some problems with the Cinnamon menu, hovering over submenu items causes the whole menu to change colour to a bright blue, and many submenu items disappear, but their name is displayed at the bottom of the menu so it still possible to use. Not found a fix yet. I also found that the terminal wasn't displaying any text, but was able to fix that in the terminal by ... edit > profile preferences > colors and unset "use colors from system theme". I don't know if these are my special problems or they are more widespread. haven't seen anything on the web. I hope you have better luck that I did. Cheers, -- Regards, Terry Duell