The balancing of the insane difficulty definitely needs some more work and will be improved based on the get feedback I get about it. For the next upload, I decreased the DEF values on that difficulty, to ensure that people can actually damage the enemies.
You get ~2 levels of experience for a squad when you finish the entire arena.
The difficulty doesn't change the rewards, which means you can rush through on the normal difficulty if you don't want a challenge but just quick rewards.