Confirm: Leaving the radiator cap off, heater full on and rev the engine around 2000 - 2500 rpm following the initial coolant fill is all you need to do. The air will work itself out of the system. Some cars will trap air and require a vacuum filler but the Gen1 CX-9 does not.
To be fair if you leave the radiator cap off the radiator may push coolant out during the initial heat cycle. For a small investment of around $20 you can buy a coolant spill free funnel (Lisle makes a nice kit) to keep the job clean. The funnel has lots of capacity to hold the hot expanding coolant.
Also if you only added 1.5 gallons before it was full that only means you only drained 1.5 gallons - so don't worry. Now by the time you get done you will have drained and refilled 2x. The result is minimum 75% brand new coolant.