Even Muhammad Mustafa (saw) couldn't unite Ummah, how come that a Prophet of Israel will Unite the Ummah? Muhammad Mustafa (saw) was the greatest Prophet (saw) the Chief. He was sent to whole mankind!
Peace Numb
When you take up an issue with us make sure you're not going to compromise your own religious stance because according to my understanding even the ahmadis believe that Masih will unite the ummah, even though you interprate this as a chain of khalifas but your intent is to unite the ummah. So you see it is not unreasonable for us to present this argument from hadith that this will be so.