Nice one!

I am distinctly underwhelmed in general by the XM4.

Have a look at Technics A800/AZ60/AZ60MK2/AZ80, all support simultaneous use of LDAC and multipoint. I recall something else working with both simultaneously too, possibly Huawei TWS.

They do step down a quality tier to do so, and low bitrste LDAC is generally considered poor, but it's otherwise all there and possible.

LDAC and Multipoint do in fact mix, just not on Sony products.

It is in fact a real thing.

Sure, it can be useful for bypassing staggered rollouts of app updates.