It does - gtkmm2 is now stable, with 2.2.x releases now the order of the day. This question is only here for dubious historical reasons, and because it makes the FAQ look bigger and more important.
At the current time Towel will only play through OSS (or ALSA's OSS emulation), but now we're using GStreamer as our backend, it's only a matter of adding the code to allow the user to select which of the available GStreamer output devices they wish to use before you'll be able to use your favourite audio drivers/sound servers with it. Depending on if there's a GStreamer sink written to go with it, of course.
Well, actually, at the moment it's being developed quite quickly. This may change in the future.
Obviously the answer to this question is dependant on what X and Y actually are. For example, we currently have no plans to support seeking within tracks, although we might add it in the future, but CD Audio support is extremely unlikely, as we believe it's a task which is best handled by a dedicated program. If you really want a feature added, let us know (by towel-user mailing list or SourceForge tracker) and we'll think about it. We'll think about it even harder if you have a patch which implements it.