>>534550365
I can't really say since I played 5.1 and 7 only. Seems 6 is already on the OGSR engine while 5.1 wasn't.
I'm guessing they added extra guns, armour and quests in 7.
Yes to the spoiler question. There's two at least (I've only gotten to SoC Pripyat and didn't visit the CoP maps).
One thing I liked about NA7 was that you could teleport by using demo_record 1, flying somewhere then pressing Enter. Makes some of the backtracking less of a pain. Also helps with repositioning if the mod decides to spawn an armoured personnel carrier right outside of a lab exit and you don't have explosives on you. There's also a spawner so you can fuck around from the start even if an AKS74-U can carry you until the brain scorcher.