Yep, those parts will work well. Shifting will be just fine- my wife's bike has the same parts (HG50 cassette and Deore M590 derailleur), and it works just as well as the original all-Ultegra group the bike had on it originally. Long cage is fine, it just means that the capacity of chain takeup will be greater than medium cage or short cage, 45 tooth versus 37 tooth versus 27 tooth, IIRC (largest chainring + largest cog - smallest chainring - smallest cog). Most mountain bike derailleurs are long cage.

No- SRAM won't work, the amount of cable travel for each shift is different between SRAM (or Campagnolo, or Shimano Dyna-sys) and the standard Shimano derailleurs. Stick with the Deore M591. It's relatively inexpensive, it works well and it is a good match to the cassette and the shifters.