I used sparrow for months and really enjoyed it but eventually had to switch back to mail for exactly these reasons. I agree whole heartedly with all of the points against sparrow but I'm not sure about the solutions...not that I have better suggestions.

Either way, I won't bother opening or even updating sparrow until it gets a universal inbox. I love the UI for the most part and I'm anxious to switch from mail.app but for now the problems sparrow introduces are worse than the ones it solves.