1. You can definitely return to broken valley and sentinel island, but not all parts are accessible only. You wont need or able to reach the lower ground areas anymore as its poisonous. Beside there is no need to do so as your current quests wont need you to do so.
2. Yes you can. You still can access the shrine on another entrance to your battle tower. From there you can run to some empty location which allow you to take your dragon form
