Clearly, Riker left at the end of NEM - he had the four pips & everything.
And I don't think Picard would just Ro again after the events of 'Preemptive Strike'. Though the DS9 Relaunch has put Ro to good use in the Bajoran Militia (and later back in Starfleet thanks to Picard's influence).
Depends on when Riker becomes a Captain. Remember the premise of this thread is that he accepts promotion earlier than NEM, one of the numerous times it was offered. So my scenario presumes Picard needs a new Exec prior to "Preemptive Strike."
For what it is worth, I never liked Shelby either. Frankly, I think it had something to do with the actor.