1. I don't think so, since every boss is hardcoded into going to the next level when finished...
2. Only if you use a boss that doesn't require an "Activate Boss" event. I think the Queen Boss is the only one that works normally without using an Activate Boss.
3. Yes to the 5 Devans, I'm not sure about the Devan Robot without the boss, but if you did, the Devan Robot boss would do nothing, since it requires both events to function properly. (And, since it just stands there anyway) The robot boss also works without the Devan, but it just floats there.

EDIT: After looking at it on my own, I've found that using the Devan Robot Boss without the robot causes most unusual side effects...
EDIT2: But what you could do, is "trap" the robot in masked tiles (more effective is place it in the center of a 3x3 square) and activate it then. The robot won't be able to move, nor will it make noise, and you'd have a Devan Boss, and 5 Devil Devan bosses. If it matters to you, you could also cover the robot using layer 3.
EDITLAST: Finally, if you use the Devil Devan boss, that Devan will be the only one that teleports, and not the Devan (Robot Boss).