What is the ruling on my marriage, and is it valid if my father apostatized from Islam in the past, then returned to prayer and fasting without returning to the Two Testimonies (Shahadatayn)? And what should I do?

Your preoccupation with these suspicions is a whisper (waswasah) that leads to grave consequences, the first signs of which have appeared in your question about the validity of your marriage contract and the validity of your father's Islam. Therefore, cast away these whispers from yourself, for your father's Islam is sound and firm, and your marriage is valid. That which is established with certainty is not overturned by suspicions and conjectures.