Part of the difficulty in projecting whether or not something like this is possible is that the cap is unknown at this point. It's projected at $89 million, most think it will be at least $92 million, and some as high as $96 million. The higher it is, the more room the C's have (although KD and Horford salaries would rise as th cap goes up, but they wouldn't eat up all the increase). It's a tight squeeze at an $89 million cap to fit in Horford and Durant, since we stand to have a pretty large cap hold for our draft picks. It's much easier at $92 million. At $96 million, it might be possible to keep Sulinger.
This matters for your desired Butler deal because the Butler deal must come third in the sequence of events. At the same time, you need to keep that cap hold for the picks you're sending to Chicago for Butler.