Can't comment on the BB lengths yet (the bike goes in this weekend to evaluate BB lengths and chainlines for the purchase of a new crankset), but I would recommend getting an extended clamp (unless you braze-on, which may make things more difficult) like the one from K-Edge (assuming that's where you are having the RD modified). I think this moves the FD about 2.5mm outward. With the belt drive, I'm pretty sure the front and rear BB lengths must match (a chain drive gives you a little wiggle room on this).

Enjoy that DI2... No more frequent cable adjustments, and once you get the system dialed in for your bike shifts are spot-on every time (and as a stoker on a good day I can hear the motors just before the shifts so we don't need to communicate them).

More on BBs when I get the info.