Regarding stage-width, etc, I think its just best to experiment. Whether that means pulling the speakers forward, closer to you, etc, to get that wider angle. I'd place for optimum imaging and stage. FWIW, sometimes adding treatments allows you to find that optimal position, because sometimes the destructive reflections at those spots would have you decide against it.
You can even try extremes when experimenting as points of reference. One extreme being with no regard for performance, but only for looks/convenience, and the other extreme being performance only, door and space be damned. When you get this reference, you might more easily find the perfect compromise. I don't always use extremes as points of references, but sometimes I do for more difficult things. Like phase for a sub, or video calibration as well.
Re sub vs treatment, boundary meetings are usually good spots to treat, no matter where your sub is located. If there was a general opinion, its that typically the front of the room gets the most benefit, if there was a general opinion to begin with.
Even if and when you get treatments, Id also experiement with placements of those. Get some stool or box or anything to help prop them if you need.
Remember every room is different, and everybody is different. BTW, I think 35% room length for viewer positioning is a very good starting point (which I believe is what you have described). Could in fact end up being the ideal.
Remember to enjoy the process. You are already way ahead in the game compared to most folks just by even considering acoustics.
-jostenmeat
