On the topic of dinkies (what a silly word): there's no way that Greenbush or Kingston become full-time shuttle services. (IMO)
What I could
maybe see is
supplemental short-turns, probably terminating at Braintree, with timed cross-platform transfers to Middleboro trains. I once sketched out a timetable a Greenbush <> Braintree <> Kingston dinky filled in the gaps between through-running services, with transfers at Braintree. In theory, you could supplement Middleboro trains in the same way. The advantage of the "boomerang" approach is that you don't have to block the tracks at (for example) Braintree while you turn the train; your "inbound Greenbush supplemental" turns into your "outbound Kingston supplemental".
And as for forced transfers between CR and Red at Braintree: the transfer isn't particularly close (1200' from mid-platform to mid-platform, as I reckon it, down and level and then up a level, probably a 4-5 minute walk), and the CR platform is only accessible via a
grade crossing, which gets dicier the more frequently you're running trains. (And unfortunately, the "ideal" boomerang service pattern, between the two lower-ridership lines to Greenbush and Kingston, would
have to use the track that the grade crossing traverses. The Middleboro Line, at the very least, avoids the crossing.)