Thank you for keeping the thread alive.
You wrote: ‘I'll assume the first half is supposed to be somewhat parallel. So, ...
The clear, swift energy rises to the headtop,
The breath sinks to the dan tian.’
You are right that the first and second parts are parallel. The rising and sinking are corresponding and inter-related actions.
I’m fairly certain, though, that qi does not mean breath here. The sinking of the qi to the dantian definitely affects the manner in which you breathe, and vis-versa, but the sinking of the qi is a more inclusive prescription, having to do with more than breath itself. Most taiji writings use other terminology when the object is breath or breathing. For example, in Yang Chengfu’s tenth essential, he states, “When practicing slowly, the breathing (huxi) deepens and lengthens; the qi sinks to the dantian.” He pointedly refers to breathing (huxi) and the sinking of the qi as distinct but related actions.
I have a lot of thoughts regarding the meaning of xu ling ding jin, but I’m interested in seeing input from others.