I believe you need to be an MNA to be PM. Even Senators aren't allowed to, leave alone an MPA, which Shehbaz is. He may end up getting into the NA on a by-election, and then be elected PM, but that can't happen immediately. Someone already in the NA will be PM, at least for the time being.
But with the N in PMLN gone, the party will disintegrate in all likelihood. How they will manage to get a majority behind one MNA, when there will be several who believe they have a shot, is anyone's guess right now.