Certainly any local can (and really should) correct me, but my recent re-reading of this thread, combined with my independent research suggests that the answers are:
1. High Roller (which is actually a cart) is the best, but the Lobster Shack is good too (and byob), and Eventide is also good (though it serves an unconventional lobster roll)
2. Eventide seems to be the preferred option for oysters and I haven't seen any place that is as universally well-reviewed, so I don't really have a secondary suggestion on that one.