Good questions.
1. The voltmeter itself uses current. If you leave the voltmeter on Bat 1 or 2 for long enough with the battery switch off it will discharge the battery. I believe the same thing can happen to the APU but it is preferable to discharge the APU battery over the main ship batteries.
2. I don't know what the main batteries have to do with the APU start except that in EMER ELEC CONFIG the APU ECB is gets power from the DC ESS BUS during APU start.
Just remember that the electrical system diagrams in the manuals are abstract simplifications of the actual electrical system. Just because a relationship isn't shown doesn't mean it isn't there.