Great discussion, thanks Andy!
About the NI protocol, of what i have understood so far: an accessory is declaring and advertising itself over BLE. For example Accessory can indicate its “preferred update rate”, but no guarantee iphone would follow. IPhone is collecting those advertisements over BLE or it can also collect that info from other out-of-band mechanism and then, it is the iPhone who decides the set of parameters and to which accessory it will range. Iphone transmits this set of parameters over BLE and then TWR begins.
It is very important to understand, that in terms of configuration, accessory is always getting instructions from iPhone. It is also important to understand the target of this Apple program: the target is to bring physical user-experience in between iPhone an accessory. Nothing more.
Attempts to use the NI with 3rd party accessories program for something which it was not designed for would fail or would not give your customers a desired user-experience.
I think potentially rotation can be done on the iPhone side, so all iphones application synced and do ranging to accessories in order.
I do not see too much problems if all iphones would select different single accessory and would start ranging, then will rotate ranging to those accessories. The reason for that is different sessions, different encryption keys etc which iPhones would randomly select. But this shall be an applications on iphone which would do that logic.
Good luck