1. You can't select the audio bitrate. For many VIP Services (Netflix, Amazon, etc) you get the option to select video resolution and bit rate you want to download before adding to queue or downloading now. For some titles on some "VIP" services, StreamFab will allow selection of and downloading multiple audio tracks of the same language, that is IF the streaming service has them to offer. In this case the different audio tracks will have different qualities (different encoding method, different bit rate, different number of channels, etc).
2. StreamFab will download the video, audio and subtitles from VIP services. There is no "Audio only" or "subtitle only" option for VIP services. The YouTube PRO module allows downloading YouTube videos at audio/mp3 only.
3. The General settings for Subtitle do seem to apply to the VIP services as well. So picking English there will default to English subs. That being said, StreamFab remembers the languages you select for audio and subtitle, so subsequent downloads should have the same language syou had previously selected.