Best to experiment inexpensively. :)
Or hire experts like GIK Acoustics.
I would call that room more like 10 x 15 just by sight, but it will be interesting to see who is more "visually" accurate, lol.
Window glass is a hard surface and can generally cause a beaming effect in the upper mids/highs when the waveforms "plane" off it. The drywall on the other side does not have as much of this effect. That being said, the speakers you have are on the floor so the first reflection points are going to hit the drywall underneath the window. However, the Usher's are 48" tall and the window might cause some un-evenness (the right side of the room might sound brighter). If you wanted to make the room more acoustically even, I would put some heavy drapes over the window and then put that comforter on the left wall like you said.
Jackets in the closet won't really absorb much bass, or even mids/highs, unless they are the really thick winter coats. Putting stacks of blankets on shelves in the closet may help absorb some bass frequencies (and also act as a broadband acoustic absorber). If you are serious, you may want to look at fiberglass/mineral wool based panels (with membranes if you want more bass absorption). Like others have said, contact GIK if you are serious.
You're listening position is pretty much almost at the back wall, so I don't think you'll have many problems with bass due to room nodes (which is more common when the listening position is somewhere in the middle of the room). If you're happy with bass response, I'd say go with it.
Fwiw. I'm still moving in and finishing a rewire on the house. The shells are only on the floor as I still have to move my ushers in and it something of a mock up/temporary setup.
Its 10.5x15.5 + Closet so 18 ft. deep
drapes are definitely going to go up. I have been in the house for 1 week while still working on it. At least a month or 2 before I'm settled