Looks like Kelli Ward has the Republican nomination for Flake’s seat wrapped up at this point. I think the only way Arpaio ever becomes senator is with McCain’s current seat, which I think is unlikely.
I think Flake dropping out actually hurt Ward’s chances for the nomination. It was looking like she’d beat Flake easily but now there’s a lot of time for new challengers to crop up. Maybe someone more moderate than Ward. Maybe someone Trumpier. Maybe even both.
Plus Ward may still end up with McCain’s seat.