"Voters will be asked to decide whether the city should be run by a directly-elected mayor – similar to London"

Nothing like London, where the elected Mayor has powers over 32 individual council boroughs, a Manchester or Salford Mayor would have powers over one. It would be accurate to say Manchester could and Salford will have an elected Mayor with powers similar to those in the boroughs of Lewisham, Newham Hackney or Tower Hamlets, but not London as a whole.

"If residents vote ‘yes’ in May, Mancunians "

If they yes the vote will presumably be void, given its an either or question.