Is the repentance of one who abandoned prayer, and repented for the purpose of marriage, valid? And what is the ruling concerning his marriage contract according to the opinion that considers one who abandons prayer an apostate?

As long as you have repented to Allah and returned to observing prayer, do not pay attention to the whisperings that make you doubt the validity of the marriage contract. Your repentance does not invalidate the contract, even if it was for a worldly purpose, such as a disbeliever embracing Islam for marriage, as happened in the story of Abu Talha and Umm Sulaym.