I'd still feel like center-running bus lanes the entire length of Comm Ave from Packards to Kenmore are still quite helpful for fixing the eastern tip of the 57 corridor. If there's enough room for an 8 lane stroad with light rail, I'd say thats plenty space enough for center-running bus lanes. Those streches and sources of delay feel like they make up a large enough portion of the entire Comm Ave segment its best to swallow the pill whole and just do center running bus lanes the whole way, We wouldn't do a half hearted combo of light rail and bus sharing a reservation (if the 57 shared space with the B's trackage on Comm Ave, it'd be the entire length of Packards to Blandford).
It seems like the center running bus lanes on Columbus Ave. were successful enough they warrant expansion onto Blue Hill Ave. Why not expand this treatment onto Comm Ave for the 57? Comm Ave, is already wide enough to cross a pedestrian, so narrowing car space as much as possible is helpful for pedestrians and cyclists with the use of center-running bus lanes on Comm Ave.