> Is there any actual list of the Metadata fields with the exact spelling/syntax?
The closest thing is probably these files in your ThumbsPlus "Data" subfolder:
xmpnames.dat
xmpnames.dat2
xmpvalues.dat
xmpvalues.dat2
I don't know what they do exactly, or what the differences are between the .dat and .dat2 files, but they seem to contain official metadata field names.
Alternatively, when you have the Metadata panel open in TP10, you can select an item and its source will be shown in the lowest pane. For example, in the Camera section, I choose "F-stop" and the lowest pane shows "exif:FNumber".