1. I do have plans to implement start and end points for playback, either in the form of minute/second markings or the already-implemented "markers" you can place in an Alda score. I agree this is a crucial feature -- it will be very useful when working on one small part of a large score. Eventually you should be able to do something like: "alda play --file my-score.alda --start 0:30 --end chorus"
2. Not yet -- I'm hoping we'll implement a way to generate LilyPond ( http://lilypond.org ) typeset scores from Alda code.
3. My philosophy on bar lines (and the reason why Alda ignores them as if they were comments) is that they're essentially "invisible to the ears," and so they don't serve much purpose except as a way to visually organize your score file. However, in order to generate sheet music (e.g. by generating LilyPond code), we will probably end up making them significant to the parser after all. I can see how it might be useful to think of your score in terms of measures, e.g. "play a few eighth notes, then rest until the end of the measure," so I'm certainly open to the idea of implementing measures.
4. I agree, although I'm a Vim user :P