I have now 12*6 combustion register at bottom of door, I will add another one at the top.
I also will add a return vent in the main space as you suggested.
As for the gap between the registers and the duct, I will make the guy screw the nails all the way through the duct , so the drywall will be between the register and the duct tight. is this ok way to do it, does it matter if the drywall touches the duct.
I appreciate your input very much.