Is seeing the deceased mother in a dream marrying off her son to the girl he loves, after repeated Istikhara prayers and the heart’s inclination towards that girl, considered evidence for completing the marriage with her?

There is no necessary link between Istikhara (prayer for guidance) and dreams. The preponderant opinion is that one who performs Istikhara proceeds with the matter afterward unless Allah diverts him from it. The fundamental principle in choosing a wife is prioritizing religious commitment and character, as the Prophet (peace be upon him) said: "So win with the one who is religious, you will prosper." The remedy for love is for a man to marry the one he loves, if possible, as the Prophet (peace be upon him) said: "We have not seen for two who love each other anything like marriage."