First of running the bows off of channel 3 and leaving the fourth empty is not advised, it will more than likely create an instability. Second, even if it did work you won't have any balance control that way and it will sound funny up front. Third, there's no benefit as you couldn't run anything off that one channel anyway. And yeah while it does increase rms it will be split between the two speakers, so again no benefit.

Wiring the four main cabin speakers in parallel to the two rear channels and the bow speakers off of the front two is the standard and accepted way to do it and will give you best results without issues.

So your running two of the JL G1300 amps for the towers? Parallel wiring two speakers together should give you a 2 ohm load on those as well.

As far as the subwoofer goes what is the rating on that? What is the amp rating for the sub? All that will determine how it's wired.

Also, with four amps shown, how many batteries are running to power the system?