Is there perchance any sort of trackage map for the proto-green line, (like, showing turnouts and where things are multiple track, not just routing) for circa 1960... or even anything as late as about 1975?
Yes.
Oh, you want to know where! At one time the Boston Street Railway Assoc (https://thebsra.org/bsra/) sold a Boston Track Map package. The package had 1887 Horsecar Map, 1907, 1915, & 1939 Boston Elevated Railway Route Maps, 1961 & 1973 MBTA Track Maps. I am not sure if they still carry it but you can drop them a note.
JoeD