Diffusion to work effectively requires space between the diffuser and the listening position, the space needed varies depending on the type of diffusion. I highly recommend you purchase the “Master Handbook of Acoustics” by F. Alton Everest. I built multiple absorbers including Helmholtz resonators (as outlined in the book) that made a significant impact on the mid bass in my room.
Remember that absorbers work more effectively when they are not placed directly on the wall. A two inch thick absorber should be mounted 2” off the wall for maximum efficiency a 4” absorber would have a 4” gap which effectively makes it an 8” absorber, assuming the absorber is open backed. I also recommend placing absorption over your listening position. It’s space you will not lose in the room and is often overlooked. The ceiling is a critical spot in the room to treat and uses ZERO living space.
In my experience getting the sub out of the corner is beneficial. In a small room you don’t need the added efficiency of corner placement. I ended up with two subs in my room. One on the floor off center but between the monitors and another about 40” off the floor behind the listening position.
As for wiring your REL. The vast majority of bass sounds/instruments are recorded in mono not stereo, you don’t have to concern yourself with stereo bass or wiring from one channel to the sub (although I’m certain somebody will object.)