1) Technically, yes, but practically, no. If you keep two different versions on the same machine you can launch them separately and they will open, but changes made to the preferences, settings, profiles and scenes in one version will affect the other as well, and the same for any hotkeys. So for certain very specific situations, you might be able to get two copies running at once and do separate things with them, but this arrangement will not be fully functional because there will be things it cannot do.
2). No, OBS is a video streaming program. You could set a very small frame size and send just a blank image, but OBS streams RTMP and I believe RTMP always has a video stream, you can't stream just MP3 or ACC. You're better off using something like IceCast or NiceCast for an audio-only stream. There's no way to tell OBS to not send video. You can leave out a source, but there will still be a video stream in your output. What platform are you trying to stream to?
You may also find that your version of MacOS may be too old to run the latest versions of OBS.