Has the girl's father's withdrawal of his approval for the engagement after affairs were facilitated by istikhara become an indication that it is not good for him? And should he forget her, or wait for summer?

As long as you have sought guidance from Allah (istikhara), you will not be lost. If you are hesitant, you should consult people of experience and righteousness, especially those who know the girl's father, so you can understand the intent behind his answer and act accordingly. For whoever seeks guidance from Allah will not be disappointed, and whoever consults will not regret.