I was wondering if you think I could make the register that's in-between (H,I) in a cold return. Do you think it will draw enough from (J,K)
The whole point in adding cold returns in (J,K) is to help naturally draw heat from the hallway feed vent (H) under your closed bedroom doors to add heat to those two rooms. In order to use (H) as a return vent you would need to put feed ducts in (J,K) which would make those rooms way to hot since you have those doors closed, which you wouldn't want since they are bedrooms.
Also would like to ask if anyone thinks I would need anymore registers in the floor space in (F) to help the heat to rise.
Chris I would leave just the one vent for now and see how it works for you. Since you will be having a duct system installed to feed the vent in the hallway (H) and using something like this http://www.airboosterfans.com/duct-fan.html
to help pull the air upstairs you should be ok.
You are already putting money into the new duct feed and return lines with fans in them, I wouldn't spend any more money until you know for sure that it's needed.