IMO.....
I would get another amp to run the sub. The main thing to remember about sub amp power is it is about consistency of delivery of the power and the amount of control it exercises over the speaker it drives. You are right in your assumption in that it is the way an amp delivers its power rather than the claimed power output or peak power output. People love to quote Jap amp peak power output - it is tosh. Quality amps deliver a steady feed and can cope with changes in draw on the system. For subs, I don't think you an go wrong with JL Audio from budget to world beater. Take a look at how many of the pros use JL Audio subs, it is usually 90%+....
As for rear speakers, you do not need them for a well rounded sound. At best, they help with a bit of rear fill but this can be run off the headunit at very low volume. Where do you stand at a concert? What way do you ears work most efficiently? From what angle is music recorded? All are in front of you and if you want to get the imaging and sound stage you get in your home world you need to spend time setting up the sound to feel as though it is coming from the front.
Finally, as for the battery, it is an issue in the Zed - you can get good power distribution blocks that you can run your current power cable to and then distribute it to the amps. I have 850w RMS of ooomph and a good few amps of current draw when mine is on full chat. Ideally a split charge and a couple of yellow top batteries is the way to go but that gets pricey and heavy!