Is a marriage considered valid if a person repents from reviling the religion and abandoning prayer, but does not seek forgiveness or utter the Two Testimonies specifically for the act of reviling the religion before marriage, while being certain that they will not return to reviling the religion after repentance?

If a servant repents with a sincere repentance, Allah accepts his repentance. Repentance is by abandoning the sin, regretting it, and resolving not to return to it. So, if you repented from abandoning prayer and blaspheming before your marriage, then your marriage is valid. Your repentance does not require uttering the two testimonies or seeking forgiveness for a specific sin; rather, general repentance suffices, as Ibn Taymiyyah said: "Whoever repents with a general repentance, this repentance necessitates the forgiveness of all sins, even if he does not recall the specific sins, unless there is a counter-argument to this general one that necessitates specification."