Do we now? I wasn't aware of that.
Anyway, I heard somewhere that the Nord armies were called back during the Oblivion Crisis, to deal with their own gates. Don't know from where, though. If it true, then Redoran would probably still stand, or at least, kneel.
True. I to think that the Redoran will probably will survive, altough in a very weakened state. I don't think they will be strong enough to really stand a chance a against Helseth.
The events you are refering to can be heard in conversations between people in TES:IV
There is grave news from Skyrim. From Falkreath to Windhelm, the foul daedra attack the land.
I hear all of Skyrim is under siege. The daedra lay waste to the Old Holds, and there is little to do to stop it.
There are foul tidings from Skyrim. The Greybeards speak of the end of all times.