Making dpdt relays from 2 spdt relays. Possibility of short circuit?
Take a close look at your motor reverse circuit. When one relay is activated and the other isn't (that is the short-time situation you are afraid of) both sides of the motor are connected to the same battery pole. With a motor, this means breaking (hard). With a normal load, this means the load does not receive any energy.

If you are realy worried about this situation, you could use a third relay to decouple your device, then switch, then re-couple it (to the switchover grid).