Thanks muchly for these replies.
After trying a few things including AVG uninstall/reinstall, I think it's a bug in this AVG release: 1. AVG is creating 2 desktop shortcuts 2. Multiplicity never disconnects or flags an error (the mouse cursor either stops at the edge of the primary or it disappears permanently if it's already on the secondary) 3. Having AVG interface open on the primary desktop causes no problem 4. It's the first problem with AVG and Multiplicity in 9 years together.
Temporary workaround for now is a spare mouse into a front USB on the secondary PCs if I need to open AVG for anything. So long as the AVG interface isn't open on a secondary, both programs work normally. Not ideal but I don't need to keep restarting the secondaries to get Multiplicity back. With any luck, it might resolve with the next AVG program update...