I would say even that is a bit sloppy. I just say it's an alternative universe/timeline. It's quick and covers the basics. The idea of time being planet based is messy, that's why Outlands should have gotten a revamp as well since the storyline officially makes no sense (1-60 Cata revamp is after Arthas died, then you go to Outlands to fight Illidan, then Northrend to fight Arthas, then back to after Arthas died until Mists which is after Deathwing falls).
Well Nozdormu never references other worlds when he speaks about time and his control over it afaik, and the warcraft universe (the universe itself I mean) is largely undetailled and even the Twisting Nether is never properly defined so I guess they could establish time is world based if they want to.
Wouldn't be the first time in fiction.
It depends on whether they're going to explain what happened or just never bother mentioning it again after the first quest. (I fully expect the latter option tbh, maybe it's for the best.)
I think they've said at Blizzcon that Outland could be now accessed by Caverns of Time though, but they weren't sure, which conflicts with that theory, although honestly it should have been that way since Wotlk/Cataclysm.
They could just have added a few infinite dragonflight mobs here and there and thrown a bullshit line about having to ensure BC have to have happened or some shit when you get to 58 and it would have done the trick.
One thing is sure though : they're undecided about the specifics. So basically even them don't know what the fuck it's all about. First they talked about entering the portal (now red) then entering Draenor via the Chronal Spire somewhere else (lol) and then talked about investigating Karazhan in the WoD pre event (and then decided not to, of course, why do something awesome that could have added some sense to the whole story and an interesting use of one of the most fascinating place in all warcraft) so I'm not even sure how the fuck we're supposed to go on Draenor.