I buy everthing you’re saying but if I have a tremendous volume of air in the loop, water won’t circulate. When I initially filled the empty hot water heater, the air vented at the top of the expansion tank throught the air float vent. To date, I have not experienced a similar release of air from the lines when warm water was introduced to the loop back through the return. My initial thought was that the return loop would reintroduce air to the tank and purge it on the way back out but that never happened. Results improved substantially with the monoflow T instead of using the drain as a return. I believe a second pump could pull the water and trapped air through the system and out through the second vent, but I think you said that 2 expansion devices will confuse the pump rates. It might make more sense to move the second pump down to the floor of the garage to help it pull from the return manifold, as the trek straight uphill is likely the most difficult leg of the trip.