Yes for CASS. I've used US Airways the most, but have also managed to get on Southwest and Jet Blue. No issues for me.

Upgrades are probably around one year. Depends on company needs.

I don't have much detail on the interview process. I think they usually do phone interviews to judge personalities and then if they like you I think they invite you down for an in person interview...I think... There's no sim, so it's just HR and maybe some knowledge questions.

Twin otter pilots are getting a LOT of flight time. I've been over 85 hrs each month since like October. I don't know what the SAAB guys are flying.

no union.
and those other questions I think a SAAB pilot will be better off answering.