As far as I know, Buffalo Zoo has no intention in bringing brown bears back to the zoo. After their Kodiak and Grizzly bear died, they said they would not bring back brown bears. However, they did house the three grizzly cubs before they went to the Hogle Zoo. With two yards, it gives them enough space to house Anana and Luna separately. I do not think they have any plans to try to exhibit them together.
It hasn't been officially announced, but I do think Kali, now in Buffalo, will be at St. Louis for the opening of their exhibit. I'm sad to hear it's small as he already displayed a stereotypical behavior or sucking his paw when he first got to Buffalo. His current exhibit now is not that big and his behavior has stopped, but with the stress of moving and the layout of the new space, who knows.
You are also correct there is not enough polar bears to go around! When I heard Oregon and Point Defiance were revamping their exhibits I almost thought what was the point since their bears are quite old. If Anana (other one) leaves North Carolina when Lincoln Park completes their exhibit, that is a zoo with a new exhibit left empty. Also, Louisville has SSP wanting to move their bears to mate with others so that could leave their exhibit empty. The only hope for these zoos is if cubs are born in the next couple of years or when Alaska completes their renovation to house orphaned cubs, we can start getting some more polar bears in the US zoo population.