If switch part A is the wiper for pins 7, 8, 9 and B is 1, 2 and 3, yes that's fine.
Regarding earthing: you should be able to common the grounds as shown. Star grounding is only important if there are large currents flowing, as in a power amp, or high frequencies. If you do get hum, find where it's coming from by a process of elimination and lift the ground from the mains plug (this is safe so long as the units remain bolted into a rack, thus grounding all the panels) or do the RC thing.