Using Facetime and iMessage abroad

I am trying to plan for cell phone usage while in Spain for 13 days with a group of teenagers. All of the adult chaperones need to be able to phone and text one another and at least one of us has to be a contact for parents in the US.

Two of us have recent, unlocked iPhones and will use this 8 gb esim plan (French Orange, but uses Spanish Orange networks). The other person uses an Samsung Galaxy A8 and will buy an Orange sim card from the same company on Amazon prior to departure. We also have Signal as a backup for phones and texts among chaperones.

I still want to communicate with family and friends in the US, all of whom have iPhones. I have a few questions about how FaceTime and iMessage work.

-Will FaceTime and iMessage work over data if wifi is unavailable? Will the apps automatically choose wifi once we return to an area with wifi rather than continuing to send over cellular data?

-If I remove my ATT sim card, will Apple still send a call or message using my US phone number, which I have listed as the primary address in iMessage and FaceTime settings (ie. does Apple just recognize this as another address when it routes the calls and messages through its servers…it’s not connecting to the phone number in any way) Will it use the next address on the list instead?

- If my primary US number will not work as a send and receive address without the ATT sim do I need to tell my family to FaceTime or text me using my Apple ID, which is the next send and receive address on the list?

-Do I need to enter my eSIM’s French phone number in the send and receive settings on iMessage and FaceTime? If I do, will Facetime and iMessage use my piddling number of phone minutes, or is it just using data?

Thanks for your help. I hope someone who has a better understanding of how these great apps work can enlighten me.