The 12 to the 11 switch should not be a problem. As long as it is the same brand you should be ok with the spacing (shimano?) and the shifting should be the same.
As for the rear hub handling the load, I have had problems this year with blowing out the freehub on the 29er with a 20T up front and a 11-36 cassette. I believe this equates to a lower ratio than some of the 26/42 set ups I've seen out there and the torque has destroyed two hubs this year alone. I weigh about 225lbs and the problem happens on grades around +22% with technical climbing on rocks (lots of traction).
I also broke a few chains on these trails with this set up. The feeling of almost going over the bars when the hub or chain fails has now retired this 29er to being just my trail bike, and I do all my tech-climbing on my 26 again with a Mavic Crossmax wheelset. I am not sure about the hub you have, but I do have a shimano on one of my bikes and had no problems.