What is the interpretation of dreaming of snow falling in summer after performing Salat al-Istikhara regarding a suitor, and the interpretation of a neighbor's daughter dreaming of her sister's engagement to an uncomfortable person, and do these dreams hold any weight in Istikhara given the feeling of comfort after performing Salat al-Istikhara?

It is not necessary to see a dream after performing Salatul-Istikhara. Rather, the person performing Istikhara should free themselves from their desires, put their trust in Allah, and then do what their heart feels inclined towards, seeking counsel from trustworthy people. If the matter becomes easy, then there is good in it; and if it becomes difficult, then there is no good in it, as Shaykh al-Islam Ibn Taymiyyah said. Istikhara is only for oneself, and it is not permissible for one person to perform Istikhara on behalf of another.