Is seeing the groom return after performing Istikhara prayer, and what is the meaning of the word "Mabrouk" (congratulations) in a dream?

We do not have specialists in dream interpretation, and it appears that what you mentioned is an indicator of good. A dream from Istikhara (prayer for guidance) may be a sign to proceed with the matter or to refrain from it, or it may not be; the evidence is the expansiveness of one's chest (feeling of ease) or its lack. If a person performs Istikhara, they proceed with what their chest feels at ease with.

If the woman is revocably divorced, her husband has the right to take her back, and her consent is not required, as Allah Almighty says: "And their husbands have more right to take them back in that [period] if they want reconciliation." (Al-Baqarah: 228).

However, if she has been irrevocably divorced, she is the master of her own affairs. It is better for both of them to reconcile if they love each other.