Re "Im Deutschen würde ich "Danke" als ein etwas höflicheres "Das ist alles." interpretieren." and similar comments about "Thank you" being abrupt.
As I've seen here (and elsewhere), NESs tend to be less stuck on traditional salutations, closes, and other formalities in e-mails than many NGSs seem to be. The medium lends itself to greater informality than a true letter does, and is often viewed as a quick, easy means of communication. Especially given the context provided in #3, where there was indeed a request for something, a simple "thank you" works perfectly fine, and perhaps even polite, for me (AE). I see absolutely no reason to see it as impolite or abrupt.
Count me among those who have no problems with receiving a "Thank you in advance." I don't know why that should be considered impolite, especially when the correspondence is quick/brief and the request is easily fulfilled. Heck, at least it's a "thank you" of some sort! Although, even if I had written that, I would have responded with a quick "thanks" (and, horror of horrors! I might have even left out other formalities at that point. (Of course, all this is conditioned up the level of communication being used. The above is applicable to friends or even co-workers that I know. If I were writing to an official of some sort or someone unknown to me, I would tend to be more formal as is appropriate.)