This is a highly personal call, but to me it would seem that a full week on the eastern side might be more than necessary/desired. There is very little development in that part of the island, and I’m not sure how much there is to actually do. You cannot swim or snorkel or scuba on that side, I’m pretty sure—the water is super rough. No beaches, though there is some jungly stuff that seemed attractive. You might split up that week between the eastern side and the far northern bit (between Calabishie and Portsmouth, including that spit of land up in the NW corner.

We did not take any buses there, so unfortunately I can’t help you with that part. Our whole trip was in a rental car. You could probably get the best answer to your “how late do the buses run” from a hotel in Roseau.