Sorry, guess I was not clear with my question. Should I put cold air returns on the floor or on the wall? Any advantage or disadvantage to either?

So I guess the unit is in the attic and the supply and return vents are both in the ceiling and you want basically the same result as a basement install just flip flopped. The return area per cavity is not spectacular at around 50sq inches which is enough return area to roughly compensate for one 6" duct supply. The best results will come from having return air in every room other than kitchens and bathrooms which doesn't sound like an option. The next best is what you are working to achieve. The closer the return is to the center of the living area the better. You can build it out into the corner of a room or closet or right in a room and use the bumped out area for pictures or put a couple corner shelves on the sides. there are a lot of ways to make it look like it wasn't an afterthought. The garage would work and be much better than what you have now but you might look into an option to add two somewhat smaller returns using some other area of the house to make the air actually circulate to multiple areas. Be creative. As far as sealing the cavity it is a pain going in with duct mastic on a brush and different sized sticks to brush it in the corners through the small entrance hole to seal the cavity without panning it with metal. This is one other case of using your imagination and what you have on hand including using your hand. The good news is if you build the duct/ducts out you will have access to seal as you build. The comfort level with one return like you are planning will increase greatly and two in different areas would make it a lot more even. There is so much that depends on the size of the home and layout. A rather square home with one central return will have a whole different comfort level than a longer ranch style.

What the tech has suggested might work if you had a low fan speed setting on your furnace. If the air flow is too fast then you may get a wind chill effect and you may actually feel colder. I doubt adding a second higher vent is going to make much a difference unless you had a cathedral ceiling. Try an experiment before cutting another vent. You may not like having the fan running for long periods of time.

It maybe the HRV. Call the builder and ask if the heating system is under warranty. I believe these HRV have a device included to prevent air from entering unless the heating or air conditioning is running.