> 4. Would it really work for mailing lists where conversations naturally fork into many branches?

I agree with this. The idea that messages must be accessed by conversation is a bit much. Support for conversation group could be nice, but ignoring that feature would have to be allowed so that clients could implement their own views.

For example, the following structure would be crappy to force on clients:

  Label
  `-> conversation 1
      `-> msg0
      `-> msg1
  `-> conversation 2
      `-> msg0
      `-> msg1

Needing to iterate through every conversation to pull tease out the full list of messages (in order to use a different conversation-grouping method) would be ridiculous. Not to mention the question of whether or not conversations should span labels and how that should work.