I really like your email and the various notes it strikes.
If you send the email to anyone else (I think the conservative types would like it too because they'd be against over-regulation and figure ordinary common sense should tell any decent, god-fearing american when it's appropriate to ride on the sidewalk or not), I think the flow in the fourth paragraph could be a bit stronger. Because you've been talking about particular areas where biking on the street is dangerous but then you switch to talking about particular areas where riding on the sidewalk is dangerous, without being explicit. The point you're making is that the government would retain the right to exclude bikes from the sidewalk in areas that are inappropriate, but you don't actually say that so the reader has to stop and think for a second.
Another option you may want to plan for is to update the law just for children. For example in nyc it's legal for children under 12 or younger (and with the bicycle wheels are less than 26 inches in diameter) to be on the sidewalk.
Also you might research towns that are more similar to your infrastructure than Boston that have taken the kind of action you want. The only nationwide bicycling advocacy that I can think off the top of my head is People for Bikes but from what I've seen they deal more with advocacy on the federal level. Transportation Alternatives is an nyc-based organization but they're really responsive so I bet if you wrote to them asking for resources they might have some ideas.