PS version headset does not support Xbox due to a lack of Microsoft chip/licensing. You can still use the PS Maxwell on Xbox, but only with the Aux cable input to the Xbox controller.
Xbox Maxwell WILL work on Playstation as stated, many, many times before on this thread. You can connect through aux cable, or set the dongle to PC mode, but bear in mind that you will be quite limited in function. It will WORK, but you may lack things like high enough volume output, volume control, and cause a higher noise floor.
No, this would mean we'd need an additional chipset, as well as drive up the cost of Maxwell, as well as add a lot more complexity to an already complex platform. It is not going to be possible. We've decided to focus on the vast majority of other features which are a considerable improvement in most aspects over the Penrose. We understand that we can't make everyone happy, but we think our targeted features will speak for themselves.
Sony already uses their own surround algorithm in Sony Tempest 3D Audio, which you can consider direct competition to Atmos for headphones. It's also free, and embedded into the core PS5 functionality.
Again, NO simultaneous audio. You can pair BT and dongle at the same time, and when a call comes in, you tap on the power button and it will switch to the call. Once it's over it should switch back to the dongle/game audio.
To clarify one last time: You will not be able to hear both game and BT audio at the same time. We have specified why earlier.