"you can only get on his route if he isn't your ex in Rose?" that is very disappointing to hear... It would naturally make sense if Iolanthe is in love with him, she would back him in the conflict.
I am sorry to hear you are disappointed.
The thought behind the decision was that, whatever Iolanthe's feelings, she has already rejected Leopold once in order to marry his most-hated enemy. Leopold is a proud man and, after such a betrayal, he no longer loves or trusts her. He would not accept her backing.