Fixing CLAMP's anatomy would defeat the purpose of the exercise. I think her foundations are good enough that a little experimentation isn't going to introduce errors into her own style. BTW, where did you find those numbers for how many heads tall people are? Men are approximately 7.5 when not stylized.
I surelly mistake the number sorry for that but I have a manual but this generally manga I forgot to specify sorry
but manga proportion can be usually weird so this pretty depends but realistic manga are aprox the number you said.
and I usually use the Head exemple because depends of how he is talls exemple if I want a short girl I will go with 5.5 heads
or I want 6 feet guys generally 8 or 9 heads is pretty good for that.
the source of where I took the proportion for the manga is : Manga for dummy
but I also check some website in the past I don't remember the name of all
but I don't say to change her style just saying to be carefull haha
when you draw it's important to be able to if the need is here to draw with the proper proportion
but like you said their foundation is good and pretty well made
BTW this purelly my opinion and this up to the OP if she could find them usefull or not haha
anyways this not because I think they have error in the proportion the other can think they have !
and this not remove this kindla beautiful.