"Mark wanted an amicable departure with Smule, and was more than willing to help his former boss and co-workers after he left." Rookie mistake Mark. Clean break, and go do your thing. Good luck but you need to understand where you made your mistake before launching this whole David vs Goliath story.
I had the same feeling when I read how they told Mark to keep the laptop. I wish you guys all the best and hope this gets sorted out soon.
Pro-tip, hand write a nice letter to your boss/colleagues on your way out. Bingo, no burned bridges, and no risk of law suits for holding on to company IP.