Is it permissible for me to ask my friend to reopen the topic of my marriage to a person I previously rejected because he did not have a job, due to my intense desire to wear the niqab, which I see marriage as my only means to achieve, or is what happened an act of divine decree, and the istikhara prayer diverted me from it?

There is no harm in asking your friend to intercede with her brother so that he may speak to your former suitor to propose to you again. This does not contradict belief in divine decree (Al-Qadar (definition)), as means are part of the divine decree. Furthermore, this endeavor does not negate Istikhara (prayer for guidance), for a person proceeds with a matter after performing Istikhara unless Allah diverts him from it.