You are based in Rajasthan, therefore you would have correct information. The Rajasthan tourism website says that Kumbalgarh is 85 KM from Udaipur. It also says that there are buses from Udaipur. click here
The other point is that tourists dont (usually) go too see only the Kumbalgarh fort. They usually have multiple agendas and take circular routes with multiple stops. I am not a local person like you, but if I would try and do a day taxi trip from Udaipur covering Haldhighati, Ranakpur and Kumbalgarh. What is your view?