1) Twin cams used to be faster by design than single cams, but that's a gap that's closing all the time, as shown by Mathews bows.
Twin cams used to be harder to keep in tune, but modern string materials like BCY 452 have solved that, and the string stretching problems single cams used to suffer from.
The back wall is more a result of design than whether it's a sinle or twin cam, there are lots of twin cams out there with aggressive draw force curves and solid back walls.
Twins cams need a bit more tuning, but do allow you to play around a bit with rest placement, single cams have less tunability by design, and the nock travel is usually designed to run level with the berger hole on the riser. Single cams don't need to be timed (it's an idler wheel at the top, not an eccentric so nothing to time them to) but they do have an optimum position.
In the long run, they will both perform as well as each other unless there's something specific you want from the bow ? (speed, tunability, smooth draw lots of let-off?)
2) Depends on the manufacturers cam and alot of other variables, but if you went from a single cam to a twin cam, your poundage would go up as they work slightly differently and from what I've seen a single cam needs a higher poundage limb for the equivalent poundage on a twin cam.......but I'm prepared to be corrected on that
I've seen bows converted over from one to the other before and you would need the cams, new strings and cables and most likely new limbs depending on what poundage you wanted.