1_No one can tell you,since they aren't out yet.
2_I don't know the exact reason but all AMD 890FX/GX chipsets support CrossFire only,maybe that's because AMD wants users to use CrossFire only(because it owns ATI)
3_Yes,you have a rock solid board,why change it ?
4_Only if SLI is a must,otherwise i would stick with the Crosshair since its a solid motherboard
5_Let me tell you something,if you want something future proof,then you can't every buy anything,because there is something around the corner.
So if SLI is a must for you,then go with either 790a/P55,otherwise keep your motherboard.Also PhysX isn't very useful,few games use it and when its enabled you'll notice a great performance drop.
there are currently no boards out right now that have all of the features you want on one board, i was looking for the same thing couple of weeks ago. the best you can do is but a crossfire board and try the SLI hack on it. are you trying to fold of are you actually trying to SLI? here is the link to the SLI hack