Well, your battery will be always on (unless the pre-amp has an on/off switch). You could switch it via a TRS jack socket, with the OV battery output connected to the S connection of the jack and the R connected back to the pre-amp (and you use a standard TS jack lead).
Your tone capacitor should be a 0.047uF, not 0.47uF.
In the mixed position, you'll need to use the pre-amps volume control to balance the two pickups. If you have to turn it down a lot, it may be worth fitting an internal volume pot to set the level when the pre-amp output volume is at maximum, to give you a wider range of control.