As for the brown dwarf possibly orbiting the Sun.... so what?
It's basically just like a warm Jupiter, maybe a few of them.
What's the doomsday discussion on this brown dwarf? Having a brown dwarf orbiting a larger star and sharing our solar system is not all that special. Well, kind of cool actually, to have one that's basically in our cosmic backyard. It's not like brown dwarfs have a history of breaking the laws of physics and divebombing planets after deciding to drop out of a perfectly good orbit, and short of that, what kind of doomsday scenario would a brown dwarf play into? 
If there's a brown dwarf in our solar system, it's lukewarm at best (compared to other Brown Dwarfs, at least), and probably not much larger than Jupiter.
Keep in mind a brown dwarf was discovered recently, with an overage thermal output hovering around a balmy 100ºC.