Screwballs? Dude, no one has thrown one of them in decades, especially a lefty. Lefties don't even care to throw circle changes usually and that has the same action as a screwball (while not destroying the pitchers arm).
If you are afraid that a screwball ruins a pitchers arm, consider Warren Spahn. He was not a "crafty lefty". Spahn threw everything, including a screwball, for 21 seasons, or until he was 44. In his late 30s, he threw the fastball only as a put-away pitch -- I remember a national broadcaster saying, "Spahnie still has that fastball when he needs it".
Don't believe in the screwball? Let Corbin throw a cutter, which behaves somewhat like the screwball.
When Corbin can't get guys to swing and miss his slider, he collapses. He needs a couple more pitches.