Based on tax laws, your marital status at the end of the year is the marital status you will use for your taxes that year. So in your case, you will file single if you were divorced on or before December 31st, 2017. Your other question is an immigration question which unfortunately we cannot answer here, however, you can call The US Citizenship and Immigration Service at toll-free 800-375-5283 and they should be able to help.
