I wanted to add some online co-op, but I wouldn't have the resources to fully animate the other player. Maybe some crazy "Your friend controls the enemies" sort of asymmetrical multiplayer mode? Maybe, maybe...
About the secret, you'll see how the bosses will work. Any kills, regardless of if they were your fault or not, count, though.
And thanks for the bug report, I'll look into it right away and have it patched in the next version (coming soon)!