Filing tax return jointly is not mandatory but it's strongly recommended from immigration point of view, unless you have much more supported documents that you are living together as husband and wife.
Sworn affidavits are a must have... very important.
You must submit I-751 within 90 days before expiration date on your card... don't miss this window.
There may be an interview or maybe not, depending on supported documents you submit. Insufficient evidences always result in a one year extension of your existing green card, so it's worth paying more attention to details. Good luck.