1 you don't, its just that if you want to be able to shoot half decent video, a point and shoot won't cut it. A DSLR can do both (if you invest in the right lenses and stuff).
2 not sure what the exact difference is in image quality and all (obviously a dslr is more depth of field and you have more control over all of the settings) between them. A dslr is only worth it if you invest like $1000+ in good lenses and tripods and stuff.
3 For mac, you can get either fcp or adobe cs6 (or vegas I think, i dont use vegas so idk) and on windows you can only use adobe or vegas, not fcp. I believe that fcp is the only one that only works on one OS. Also, obviously a more powerful computer is going to be faster and work better for editing