Yes. According to the IRS:
You generally can change to a joint return any time within 3 years from the due date of the separate return or returns. This doesn't include any extensions. A separate return includes a return filed by you or your spouse claiming married filing separately, single, or head of household filing status.
Once you file a joint return, you can't choose to file separate returns for that year after the due date of the return.
Yes. According to the IRS:
You generally can change to a joint return any time within 3 years from the due date of the separate return or returns. This doesn't include any extensions. A separate return includes a return filed by you or your spouse claiming married filing separately, single, or head of household filing status.
Once you file a joint return, you can't choose to file separate returns for that year after the due date of the return.