For context, LDAC is one of the few wireless audio codecs stamped Hi-Res by the Japan Audio Society and its encoder is open source since Android 8, so you can see just how long Windows is sleeping on this. I'm excited about the incoming next gen called LC3plus, my next pair is definitely gonna have that.
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I could well be wrong about the AAC passthrough, and I should have hedged that statement with "allegedly" as I've not tested it myself.
To your other point though, I disagree - there are plenty of ways you could pass through an unchanged AAC bitstream, but still mix in other sounds when required. For example, having the sender duck the original bitstream out temporarily and send a mixed replacement bitstream while the other sound is playing. Or (and this would only work if you control the firmware on the receiver, but if you're using Apple headphones with an Apple device, that's not a problem) sending multiple bitstreams to the receiver and letting the receiver mix them.