I assume that there are no children living with you.
Your filing status is Single. If you were divorced in 2016, you cannot claim your ex-spouse as a dependent under any circumstances, even if you lived together all year and you supported her.
If you were divorced before 2016, you lived together all year, you supported your ex-spouse, and your ex-spouse had income less than $4,050, you can claim her as a dependent.
You cannot file as Head of Household based on claiming an ex-spouse as a dependent.
If you are divorced she isn't your wife. You are single. You file as a single person. She is only your roommate. You are either married or you aren't married. IRS doesn't care about the details.
You file as a single person.
It's exactly the same as if you had a friend living with you as a roommate.
The state only cares about your current legal marital status. Not who you live with, who you're sleeping with, how you and your ex get along, etc. For their purposes, they only care that you are a single person.
If you're legally divorced, then you file Single or Head of Household (if you qualify).
Her moving back in doesn't reverse the divorce.
If you are divorced and have not remarried, then you file as single.
She is not your wife. She is your girlfriend/roommate/booty call. Take your pick. Under all these variants, you file as single.
In the US the IRS doesn't have a lot of nuance or gray areas. You're married,or you're not.
if your divorce was legally final you are single
if one of you has a child to claim that one can claim head of household