I'm sure others will address your other questions, but I'll tackle the design/volume one.
You are fighting two different design challenges with box design and volume.
Box design (mainly the width) determines the baffle step. An on-wall or in-wall speaker will have a minimal effect here, otherwise, the narrow speakers have a very high frequency rolloff, whereas a wide speaker would reinforce the bass more, but then lead to diffraction effects. The corners of a cabinet can act like re-radiators of the sound. For a narrow cabinet, the path length differences are small, so this does not affect things much. For a wide cabinet, you can get a certain degree of comb filtering in the higher frequencies. So you end up designing the narrowest cabinet that you can while trading off the bass reinforcement.
(note: you can get that bass back by using the shelving filter or BSC circuit in the crossover to trade efficiency for bass extension -- basically lowering the efficiency of the higher frequencies to match the lesser bass output)
With volume, you are trying to tune the "air spring" behind the woofer to smooth out its extension. This will be dependent on the number of woofers, which models (and their T-S parameters), and whether the speaker is sealed, ported, or passive radiator (or .5 way design).
Sorry if I raised more questions than I answered -- this is a complicated hobby
