They don't need the exception to "lower Yao's salary." Yao's salary is cut in half by insurance. That is true of any team that has Yao. His cap number is 17, but any team will pay 1/2 of his salary. The salary dump is a seperate part of the deal.
Your point was that the Rockets would need to take back 17. Because the Kings would still be under the cap, they don't have to be within the 125% limit. The 125% applies to teams over the cap trying to take back more salary. That's why they can dump him here for a lot less. You are right, the exceptions don't cover that part of what I was talking about. But the Rockets don't have to take back 17 million in contracts.
You are right the TPE's work to make the deal work the other way. You can't combine the exceptions, but Rockets can take back Yao 17 + 6.3 in TPE ... and if they got a 3rd team in the trade they will have another 5 TPE to play with.
Thus, in just about any Yao trade with Sacramento you don't need the trade machine. As long as: (1) the Kings payroll isn't over the cap after the trade; or (2) the Rockets aren't taking back 24 million in contracts for a two team trade or 29 in a 3 teamer ... it's probably going to work.
And because of the insurance - a cap nutral deal will end up being a salary dump for the team that takes Yao.