Quick jumping out of order for the questions, of course these are my opinion, and you will find many varying opinions on many details.
Q2: Not the big rotund/arc'd diffusor. Stick to flat and add what ever treatments you come up with. Your M2!5 speakers won't put a lot high frequency energy on the front wall, so the priority should be controlling bass-midbass energy in the length dimension, and enough diffusion to keep the late reflections and general feel of the room balanced.
Q3: Depends greatly on likely seating distance vs speaker location vs height and angle of ceiling. I didn't catch where the peak of the roof was in relation to the 15' x 20' dimension?
Q1: Midbass and bass problems are most often the most problematic and hardest to combat in the depth dimension of the room. Devices thick enough or tuned to act below 300-500Hz and as low as possible make life much easier, and often the front or rear wall are easier to find depth for treatments rather than making the room narrower. If you're bothering to build out shelving for vinyl and media storage at the rear, I'm awlays surprised so few use this to conceal some extra treatments. Making the rear of the shelving either fabric or a purposely perforated panel with 4-8" of space behind filled with some rockwool can very easily create highly effective and invisble bass absorption in a range you can never have enough of in a closed room.
