Will I have to pay back my bonus if I ETS because of my RCP? Also, can I join the NG before I'm fully out?

You will pay back the prorated amount of your unearned portion of your bonus. Take your old ETS date at time of the last re-enlistment, the number of months you added do your new ETS date at time of re-enlistment is the amount of months. Take your bonus amount prior to taxes, divide that by the number of months. Multiply that by the number of months you will ETS early. That is the amount you will owe. You will pay the pre-tax amount, in order to get back the money you were taxed you would have to go file an amendment to that years return that you filed your taxes on.

You are eligible to join the National Guard six months prior to ETS as long as you are not flagged at that time. The National Guard does not have an RCP. I don't know how you would join the Coast Guard if you were already in the National Guard, but okay. Good luck joining the Coast Guard with any history of misconduct.

If you are able make your rank back prior to your ETS, your old ETS date will be reinstated

I've heard the Coasties are crazy strict about enlistment qualifications, talk to a recruiter ASAP to check if you're even eligible.
Your question is a little weird as well (no punctuation), are you trying to do Guard and Coast Guard?
Be prepared to receive bad news, as it looks like you'll be paying it back. You didn't complete your contract for which a bonus was awarded.

Retention NCOs will be the SMEs for these questions