Is it necessary to repeat Salatul-Istikhara to decide the matter of returning to my former fiancée, and was the dissolution of the engagement a result of the Istikhara, meaning that the matter should be forgotten, or should I try and reconcile?

We do not know whether the engagement was called off due to Istikhara or not. The benefit of Istikhara for a person depends on their sincerity in it and the correctness of their entrusting the matter to Allah. There is no impediment to you returning to propose to the girl and repeating the Istikhara concerning her, for she might be good for you now. The preponderant view is that a person proceeds with a matter after Istikhara and does not abandon it unless Allah diverts them from it.