I built my latest bike with the stem initially in the up position - making the bars about an inch and a half higher than usual. The front end was super-light, and I could actually pull on the bars and loop the bike over backwards. Flipping the stem down has returned a much nicer, more balanced and planted feel to the bike. I can still lift the front for bunny hopping obstacles, but it's not too light. So I'd agree, small changes in position can make a big difference to handling and comfort!
The widest flat bars are Salsa Pro Moto, at 660mm (26"). Most riser bars are about the same width. I don't know of any wider stock bars. If you know soneone with a lathe you should easily be able to have some extender plugs machined from aluminium and epoxy-bond them in to get the extra width you want. It should be no trouble to add an inch to either end. Anyone remember 'Galindo' BMX bar extender plugs from the mid-eighties?
Fat tYres - Conti Speed King 2.4" or Schwalbe Fat Albert (2.35"?). Both are light and supple, for fast rolling. I sometimes use the Speed Kings, but only the 2.2" version because it's not rocky here and 2.4's come too close to the cable on V brakes. Speed Kings are great. Anything fatter is likely to be a burly freeride tyre. Some of the freeride and downhill tyres are twice the weight of the aforementioned, and are too 'dead' for nice cross country riding.
"Gears are like eyes or testicles. Having more than one is an unnecessary luxury." Elvis A. Presley