In order to construct realistic possibilities, you need to stay within a reasonable framework of what the NBA would allow and what value our assets really have. First, our TPEs are not really in play for Harden. He does not fit within any of them and since they cannot be combined, they are not going to help get Harden other than maybe in the off season.
With no TPE in play, outgoing salaries need to be roughly equal to Harden. Harden is $41.2M. To get there, you either need to include Kemba ($34.3M) or BOTH Brown ($22.7M) and Smart ($12.9M), and still throw in Langford or someone like that in either case.
Kemba is currently hurt and just underwent an experimental treatment on his knee. It is going to be very hard to include Kemba in this either directly to Houston or via a third team. Healthy, it is a different story but right now, I don't see it. I have not seen any realistic 3rd team option where Kemba gets back some young players that Houston would like (or where Houston wouldn't just trade Harden directly to the 3rd team).
That leaves a trade starting with Brown AND Smart. That is probably something that Houston would at least consider, it is fair from their perspective, but someone can beat this or Houston may decide it is just not worth it and wait. But if you think we are going to get Harden, I hope you are thinking Brown and Smart.
For me, I would love the team if we got Harden for Kemba and could keep Smart, Brown, Tatum but I don't see how that happens. I am not so keen on a team with Kemba, Harden, and Tatum.