1. Correct, Zone Exclusive means that you can only have one Zone Exclusive in that zone. It's fine to have other conjurations in that zone.
2. No. when a creature is summoned it does not count as them entering the Zone. This ruling was made for balance reasons. You are also right about Poison Cloud though, since it isn't triggered by a creature entering a Zone it should still trigger at the normal time. Moving into, being pushed into, teleporting or being teleported into a Zone are considered entering the Zone.
Finally I would like to point out to you that if your opponent is clustering a lot of creatures and/or conjurations in one zone, it is time to hit him with some Zone Attacks to wipe that smile off his face.