What does 'in waiting' mean in 'deputy manager in waiting'?
Sounds like they are waiting for someone to retire (or get promoted or get fired) so they can fill the position.
Or it could be a reference to a "lady in waiting."
It's hard to know.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.