Great guide. If you are into these sort of guides of how to deal with the US government I have written a couple for the W8-BEN-E form [1] (you need this if you have any US customers) and also for registering to do business with the US government [2]. These are biased towards Australians, but they should be helpful for others too.
US customers here must mean that if you sell directly to US customers. If you sell via App Store to end users in US, then this is not needed, because Apple will be your customer (Apple Luxembourg for Europeans), not the end user.