For audio devices like speakers or headphones I've thought about an idea for playing music at least. You could have the host device send the playback device the files and the playback device would play the file locally and it would be synchronized with some sort of a timestamping routine between the host and the playback device. There wouldn't be any need to transfer data in real time in high bandwidth between them so you could have lossless audio. Being able to play back a majority of audio codecs shouldn't be all that hard or license encumbered. Even if you take a shortcut method and just transcode everything on the fly to flac. You maybe only need to store the current song that you're playing back. Heck you could extend this so it doesn't need to be built into the headphones or the speakers as a protocol and it could just be like one of those portable Bluetooth things that you can buy that you could plug any audio device in except it uses this new thing.

But the touch on all of the things that you brought up in your original post related to like real time audio. I swear there are implementations where multiple headsets on one device or multiple devices to one headset do exist. I don't think the problem has much to do with Bluetooth as a standard even though the standard is extremely complicated I think it all has to do with software and implementation on the part of the host device like the computer or the phone and the playback device. Like if there was a standard reference device example for both of these things that did everything correctly I'm pretty sure all of the things that you brought up work. Although I would like to be corrected on this.