You're making a distinction that doesn't exist. The spell explicitly allows those inside of it to use any spell and have it work. Redcloak is on the inside, so if he casts a gate, it will work.
The only Summon Monster spell we've seen Redcloak use did last for a short time. The Mammoth disappeared shortly after he entered the city, when he confronted the city's High Priest. And that was an extended spell at that, lasting twice as long as normal.
Sure it is. The purpose of the spell was to keep the bad guys from entering the area to begin with - if they're in already, its purpose has failed. Besides, it's a loophole he explicitly left by the specified summoning exception.
Zevox