Again, free internut legal advice is worth every penny. Consult a professional. But you must consider locally the context of "exclusive," that is "motor traffic." If the phrase included "motor traffic and pedestrians" you might have an argument.
More generally, when looking at the definition, since bikeways like the Minuteman are specifically INCLUDED in the scope of the chapter, you would be wise to consider that your interpretation of the definition which EXCLUDES bikeways like the Minuteman is likely in error.
See
MGL Chapter 90E, Section 2. Specifically....
Now, FINDING the regulations for the use of bicycles on the bikepaths is... ...interesting. But if you look at those signs conveniently posted, there they are. (The are *NOT* simply "etiquette.")
However, there are MANY OTHER bikeways/rail trails where the regulations come from another entity, specifically now the DCR, formerly MDC. These cover paths such as the well used Paul D. White bikepath, the Cape Cod Rail Trail, the Norwottuck Rail Trail, Alewife Brook Path, etc....
You can actually FIND these regulations at
302 CMR 12.
Specifically about the "controversial" bell/"on your left" question:
It's simple. MUPs are not sidewalks. No matter how many times people who don't ride on MUPs insist that they are.
-mr. bill