A:AnswerYes.
(1) Photos from Apple's website shows the Ultra's bands with the same "connector-interface" as other watch models' bands.
(2) Further, 9to5Mac has an article on September 2, 2022 describing compatibility:
Supposed to avoid URLs in here, so you can always DuckDuckGo/search for something like: "9to5Mac apple watch band compatable with iphone ultra?"
A:AnswerI believe the return period starts from the day of purchase (you would have to check with Best Buy for that). However the cellular activation starts at the day your activate the plan and can be cancelled at any time. I use AT&T and that is their policy.