here is my thought:
70-300l for more reach: imo, i would not choose this one... for more reach, i would wait to get 7d mark ii or 70d so that i would add another body to my list and it will be obviously more reach than what you have planned when pairing with 70-200mm. someone here might claim that 5d mark iii is better at high ISO... sure, but keep in mind that lens is f/4-5.6.... at most two stops and think about it, you can shoot at f/2.8 with focal length of 320mm instead of f/5.6 at 300mm
This applies only if you're making big prints (larger than 16x24"), submitting for publication, or have some other need for high MP, the cropped 5DIII image will have equivalent IQ to the APS-C image (and better IQ at higher ISO). The 'reach' of APS-C is often an illusion.
Plus, the OP's talking about lightening his load - does carrying two bodies make sense?
The part about the primes that kills me is to keep two people in focus I end up having to stop it down so wondering what other benefits the primes will have besides subject isolation options and compositional thinking vs zoom.
With the MkII zooms that you have, when stopped down to f/2.8 or narrower, the primes' advantages are negated, except sometimes size/weight (e.g., 135L vs. 70-200 II).
Then picking up a 270ex for a lighter travel flash.
You can't bounce this so I'd avoid it; direct flash is horrible.
Might want to check your facts before making an assertion like that...