Left house in father's will with stepmother having life use. He died and she signed it over a few years after. Which date do I use to determine fair market value, his death date or the date she signed it over?

The w=house was ALWAYS yours. What she signed over was her free use of it.

Use the FMV at the date of your father's death, or six months after the death date if the estate opted to use that date.