Okay, so, I'm not 100% sure this could work, but I think there is a way to get the nether back and duping wont be allowed: Well, you could disable every single item that you used to do the duping, then enable the nether. What I've worked out is that when you do that, duping will not be able and the nether will be back, as many people wish it was.
You can't. If you want to enter the nether, nether portals have to be activated. So if they are not, no nether.
You're saying that they can't make a warp nether? I've seen it on hundreds of servers. It's possible.
They could enable nether, make a portal, goto nether, set warp, disable flint n steels and fire charges and then people can goto the nether.