Now on to the questions:
1. 6" center to center on each tee is a pretty good distance.
2. I would pipe it in parallel off the main loop for sure.
3. I would definitely recommend a motorized mixing valve to keep your temps exactly where you want em. This way if you have an outdoor reset or some other control fluctuating your main loop temp you're not having mixing valve issues bring the temps back too hot or too cold.
4. I'd say try widening your set point differentials to about 145 and 180. Buffer tank size I would guess about 30 gal. If you have a run fraction of less than about 30% your efficiencies really drop off.
6. If by this you mean the return temp vs supply temp I would say it depends a bit on how you have your system set up. 10-15F maybe though.