To add to What
@RJB1690 said, you can accept any offer you receive and are not committed until the day you report for INDOC/BCT. If you were selected by USCGA during early action, you could accept the appointment. If you then received another offer that you found to be higher on your list of dream schools, you can then accept that and decline the offer you had previously accepted.
Understand that the schools are racing to achieve a target number of students for the incoming class. Your accept/decline decision factors into their algorithms that dictate how many total appointments they need to send out to achieve the desired yield. Service Academies are often competing for the same top talent each year and these types of situations present themselves each admissions cycle. Each applicant has a mental rank order of schools - just like you laid out in your question.
Accepting an appointment does not commit you to attending that school should a perceived better offer come along later. The 1 May decision date is intended to clear the deck of people on the fence about what they want to do next year and provide clarity to the Admissions team for how many more appointments they need to send out to achieve their target class size. Additional offers of appointments will go out right up until the report date for INDOC/BCT to achieve the class size they want. I say this to remind you that the real end date for this adventure is the report date for the school. Students should understand that the 1 May date is important and your best chance to receive an appointment comes before that date, but the music in this game of musical chairs does not stop until the report date and some late appointments will go out.
I was headed to USMMA but received the USAFA call two weeks prior to their report date. I kindly called my USMMA liaison - thanked him for all his help, informed him that I received an appointment to USAFA, and that I would be going there instead. He understood - these types of last minute changes happen each year.