Is it permissible to perform Salatul-Istikhara concerning a person who has not proposed to me, and with whom I have never spoken, but whose righteousness has become apparent to me and whom I desire as a husband?

It is not a condition that the suitor propose to the woman who wishes to perform Istikhara concerning him. Rather, she should make the Istikhara supplication. If there is good in it, Allah will facilitate it for her, and if it is otherwise, He will avert it from her.