Does supplication repel divine decree, such that one can supplicate for a man whose proposal was rejected, to become a righteous husband, despite the Istikhara prayer which may indicate evil in him?

Supplication does not avert divine decree; rather, it is part of Allah's decree. A Muslim woman may supplicate to Allah to improve the condition of the one she desires and to make him her destiny, and her supplication should be general, asking Allah to grant her what is good for her. If she seeks Allah's guidance through istikhara, then whatever Allah decrees for her is the best. She must beware of any unlawful relationship with a man.