IOS – Download iOS app IPA file from App Store

There is no publicly available data on the actual iTunes App Store download counts (except for occasional special events such as the 25 billionth app download contest, and a few individual developer reports). The exact app download numbers are considered confidential information between Apple and each developer (perhaps because an individual's earnings could be so deduced). Each developer has access only to the counts for their own apps.

That said, a few developers do report some of their download numbers; and there are various estimations of the download distribution curve based on this evidence: for instance this blog post, and this academic paper, both from 2011, which may or may not be accurate. (Disclosure, the blog post is mine.)

As for part #4 of your question, the "on average" answer may not be relavant, as the app download distribution may be so skewed that almost no app has a download count that is anywhere close to the mean average, but mostly a lot more or a lot less. For instance, whatever the big mean average number is, there are some reports that most apps get downloaded less than once per day.

No - the IPA format does not make source code inclusion mandatory, so there’s no general way to do what you ask. As alluring as the idea is, just because something is possible on Android (What you’re talking about is not in the sense you want it), doesn’t mean it will be possible on iOS.

Now, if the developer decides to embed the source code into the IPA - you will be able to read it depending on the method of packaging it since the IPA contents are well documented by Apple on the developer site and many utilities exist to decode the application bundles.