1. I don't see why you can't have spangles. If you breed an English with the 'normal' type who is a spangle, you'll get a part English spangle. As far as I know, the genes are totally unrelated, so I don't see why it should make a difference.
2. Some budgies in the wild do have the different mutations, but they generally don't survive as long (they don't camoflague as well). It's like you can get albino birds in the wild. People caught the budgies with the different mutations and bred them, so they increased in number, whereas in the wild they wouldn't have (natural selection means that the better camoflagued budgies would be more likely to live to an age where they could breed and pass on their genes).
New colours could come about if a gene mutates I think, but other than that you can't create any new colours.
That's my understanding of it anyway. Hope that helps and that I made sense.
Try not to think of budgies as 'australian' and 'english' and 'american'. They are all the same bird. The 'english' is not a variety, its simply that england was the place where the exported australian birds were most heavily exposed to selective breeding and became larger in size, length, feather etc. They are still the same bird, but every generation was selectively culled so that only the chicks that were better than the last generation were kept.
Because australian birds were exported to america also, but not as selectively bred, they remain for the most part quite small. Definately larger than their wild counterparts, but small by comparison to the english standards now established. New Zealand began to selectively breed australian birds and were doing quite well, but in the 1990's they imported around 100 english birds, and started breeding and increasing that stock and in some cases mixing it with the best of the new zealand stock. We still have 'pet type' budgies all over the place, but show birds are much much larger here.
Most mutations occured in captivity and were selectively bred to establish and secure the variety. Many mutations occur in the wild but such birds stand out in a flock and fall prey to predators quickly. I have heard of dark greens and opalines and ino's in the wild being reported but they are rare and almost never seen again. 'English' spangles are some of the largest and most popular varieties bred for show because it is a dominant gene.
All the colors we see in budgies now are present in wild budgies. Mutations simply alter, remove, or re-distribute existing colours to give a different appearance. For example blue budgies are simply green budgies who cannot produce yellow pigment (yellow + blue = green).