Going to address this with point 3 because it's essentially the same thing. Bungie is private company. It is not a publically traded company like, say, AB is.
This means that Sony can't just barge in there and buy up their shares. Bungie can negotiate its own conditions for selling shares and one of those conditions is, considering Bungie's recent track record and the fact that Sony has publically and very specifically said that Bungie will operate as an independent developer/publisher, most likely that Bungie remains full control on how it operates and what/how/where it publishes its games. These conditions are legally binding.
That does not mean that Sony can't negotiate a deal with Bungie for exclusivity in the future. What it does mean is that Bungie will get the final say in it.
My initial response that started this discussion was to another poster that said he was surprised Sony didn't pull Destiny 2 from Xbox yet when, in actuality, they probably can't.