You can only use a married status if you are legally married as of December 31. If you are not legally married, you can't use either married filing jointly or married filing separately.
The address on your tax return is only used for the IRS to send you official mail, it does not have to be the place where you live. For purposes of residency (what state to file in, where is your main home, etc.) that is determined by the actual facts. You can't change your residency on paper unless the actual facts of where you live most of the time change.
