Is a disturbing dream, which came after repeatedly performing the Istikhara prayer and calls for abandoning what is lawful, considered a whispering from Satan, especially when the heart feels tranquility towards the matter?

You did well by performing Istikhara concerning marriage to this girl. The result of Istikhara lies in divine facilitation or its absence. If the girl is religious and of good character, then proceed with marrying her. If there is good in it, Allah will facilitate it; otherwise, He will divert it from you. As for the dream you mentioned, do not pay attention to it.