I had the same question. But then, I formulated my own theory.
Seems to me that as Chittor will be lost to Akbar, and many greats, including Jaimal and Patta will be lost, and of course all the Warriors, the shaheeds too. According to what I know, Pratap went into jungles and prepped for guerrilla warfare after loosing his state. This might spread the news that even Kunwar Pratap is missing/dead, and with Amar too small, US might crown JM as the hier with no choice left. And DB can act as a catalyst in the same.
According to what I read, pratap was there during US's funeral and he was going to leave Mewar because US crowned jagmal...but US crowned JM the heir when pratap was there...I'm not really sure...so what you think could still be right...it's so confusing! Let's see how the CVs show this