Does uttering the Two Testimonies (Shahadatain) with remorse and a firm resolve not to return to what expels one from the fold of Islam reinstate one who was outside of it, and is it a condition for the validity of this that one be ignorant of or have forgotten the acts of disbelief committed?

Whoever enters Islam with certainty does not exit it except with certainty. The questioner's doubt about being a Muslim has no effect. If a person disbelieves due to a statement or belief, then their regret and utterance of the two testimonies (Shahadatayn) are sufficient for their return to Islam.