TSM will start shutting down around an hour and a half before the fireworks show. By that I mean they will stop allowing new guests into the queue and begin assembling barricades which will be put in use after the attraction is cleared of the guests who are still inside.
With Little Mermaid they will likely work out which show will be the last of the day, and not load for a new show after that one. It'll probably be at least an hour prior to the fireworks.
If they're cancelling morning FPs no idea what the deal is with that.