Ok guys, be prepared to chuckle.
When I removed the build plate to have a look at the thermistor I noticed that the color of the wiring (white teflon insulation) was different than the wiring I had going to the bed temp thermistor slot (black vinyl). Hmm.
Coincidentally, the previous "Z Stop" switch wiring was white teflon coated, while the wiring I cut at the Z stop switch to splice in a new piece of good wire was black vinyl.
I put a new two-pin connector on the white teflon wire, plugged it into the bed temp sensor socket, and now it all just works.
Coincidence? I think not. š
I don't know how I managed to mislabel the wiring as I pulled it off the old Melzi board, but somehow I did.
I have one remaining issue that I know of (at least until I try to plug in the PanelDue and 7" screen), but I'll post that in a separate thread.