Is it permissible for one person to pray Salatul-Istikharah on behalf of another, and is the vision from an Istikharah prayer performed by a pious person for me considered a proof for me?

Istikhara cannot be performed by one person on behalf of another, because it is an act of entrusting one's affair to Allah and delegating it to Him, and this can only be done by the individual themselves. It is recommended to repeat Istikhara, as it is a form of persistent supplication that Allah loves, and there is no specific number for repetition. One should not rely on a prior inclination before performing Istikhara; rather, one must leave the choice entirely open, and what a person sees in a dream after Istikhara is not to be given weight. The correct understanding is that whoever performs Istikhara truly for Allah, will only do what is good for them, whether their heart feels at ease or not.