Harsh, I think your time would be better spent rebuilding a new/replacement used gauge as the corosion is likely to be impeding proper operation. Complete disassembly of these complex instruments is often a bad idea...difficulty reassembling without some specialized tools I am betting is impossible. Would one of the speedo repair shops quote you a ballpark number if you sent them the photos? What would Marcel charge, if he could do it?
Definitely looks like it has been very wet inside the case or sat upside down exposed to rain or it was flooded. Was there a dark stain around the needle? Storing them improperly leads to the oil soaking into the face plate. They should be stored face up. Or, installed on bike...
Hard not to do a good job for your kids...
With the colder weather what do they have her doing? Still same sandblasting/ media blasting? Or, did they find a new level of hell, I mean work assignment for her?
I have a set of old CB175 gauges...wonder if they are compatible, or what they would be compatible with...
David