We are starting to draw out some built-in shelves/cabinets/etc for our new home and unfortunately in two rooms we want to put large shelves there are air return vents in unideal places. These are both interior walls and on the opposite side of the wall there is empty space. The first room has one supply vent, one return vent, and a 4ft french door with a little less than an inch gap underneath. The second just has one return vent, but is totally open to the rest of the house (it is the entryway from the garage).

We have three ideas, but aren't sure which is better or will be easier, or if there is anything we are missing:

(1) Flip the return to the opposite wall, either by removing the register box and rotating it 180 degrees, cut a hole on the opposite wall, and patch the drywall over the current vent. It looks like the return box is crimped onto a duct and screwed into the studs, so this may be difficult.

(2) Remove the back sheet metal of the return box and move it to the current opening (and probably tape/screw it to the existing box), essentially flipping the return without removing it. Then cut a hole on the opposite wall, and patch the drywall over the current vent.

(3) Cut off the top of the return box and open up a hole at the top of the stud bay (where nothing is against the wall). This would essentially use the stud bay as the return duct. We are in southwest PA, so I am not sure if this would be against code. There is a wire running through the study bay as well.

(1) and (2) would leave the rooms without a return duct. Not sure how important that would be given the first room still has a largeish gap under the doorway and 2 is totally open to the rest of the house.

(3) would probably be the easiest and keep the return in the same room, but I know using the stud bay as a return is frowned upon.

Pictures of rooms pre-drywall. Drywall is already up though.

Room 1:

Room 2:

Thoughts or suggestions?