Why, I can not think of a good reason. But yes, many sites do have 1 on top of the other (skyscrapers usually in the right or left hand side columns).
I agree, I don't see any problem with it. You probably won't get that great of a CTR though and it will look obtrusive on the site.
There is nothing wrong with placing three blocks without any interval but: 1 it will appear to be greedy 2 another better placement would be lost because the max is three 3 too many blocks will invite lower-priced ads. Because of reason # 3 I do not exceed two blocks.
Yes it is allowed. But from our openion divide it into different locations of your webpage for e.g. Top,leftsidebar,rightsidebar & bottom. This way u'll get more positive response.