His words not mine, I'm going to say it's like post second verse and the mode or style of the song sort of shifts in the composition itself. Notably Brad's take on Nathan's song (?) which amplifies my point a bit about their creative process, in SMMD you have Brad being able to bounce proper song and lyrical ideas off Nathan in a fuller- or just different- way say than with his brother and Marc. To your second point I don't think there's either the interest or economics for Marc to be a 'Barr Brother'. He seems genuinely to be engaged in his engineering work, what gigs and sessions he does work, and I guess life in New Bedford, Mass.