Setting aside Horford for a moment, if we are able to sign and trade Kyrie, we would get a large trade exception. It would not be a stretch for Brooklyn to renounce their way to enough space to do this. We would have to send out a half decent pick or something but it would be well worth it if we can then sign and trade for Walker using the trade exception.
In this way, we can get Kemba without having to renounce Rozier, Horford, and all the other FAs. That keeps open the options to sign these players or sign and trade them. That doesn't mean we bring back Horford or Rozier, but we leave open the option. Even if we are able to S&T either Horford or Rozier, it is very valuable in that we get a player back or a trade exception, thus preserving the above cap money.
It would be ironic if we were able to trade Kyrie and get an exception. Then use that exception to get Walker, then get part of that exception back by trading Rozier to Charlotte. I think it would be unlikely that we sign Horford but it would be awesome to somehow get a trade exception for Horford too. Then we could take Capela without trading Smart.