a) No, the foundation is *not* the source. It is simply somewhere which happens to archive the source for the purposes of using it to write an encyclopedia. The source is the, well, the source; the person who wrote to us with the correction. (I write articles and give the book I worked from as the source. I don't give the library I read the book in, which seems to be the appropriate analogy here...)
When you cite a book that book is a published source that can be confirmed by anyone visiting a well stocked library. When you cite an unpublished email you are actually citing the person who read the email, as that is the only link in the chain that can be confirmed (since the person reading it will have said so on the article).
The source of a piece of information, for all meaningful purposes, is the last link in the chain that you can get to without taking an unreliable step. The step from the person reading the email to the email itself is not reliable, so the effective source is the person reading the email.