Hmmmm it sounds like you are drawing a correlation between back issues and headaches. For me, I’d likely approach them as two different issues, though of course Yoga is holistic by its very nature and so of course the larger picture must be kept in mind.
It can not be repeated too often that the student has to be assessed and that no one answer suits everyone. That having been said, a student who has back issues may have issues (in backbends) with compressing the lumbar spine or with the hip flexors in the front of the body.
The first should be approached by teaching the appropriate actions in backbends to maintain space in the lumbar spine. The second can be addressed in two ways; opening the hip flexors using posture that provides gentle progressive opening (rather than demand it) and by calming that which is aggravated, shortened, in spasm, or wrought with fear (the iliopsoas).
Backbends can be very helpful for lower back issues when a) they are done correctly and b) when they are not done to excess.
Headaches are tricky buggers and I’ve not seen anything conclusive about their origin. Of course a practice that places undue strain on the neck muscles or one that allows the student to compress the cervical spine, or one that does not educate the student in the use of the serratus anterior, can increase the probability for such things as headaches.
And, of course, all physical manifestations are just that. So while we are trying to explore that which will support the student’s well being physically, we are remiss as yoga teachers if we are not sharing the other tools of Yoga - namely pranayama and meditation. I advise my students to stop at onset then go inside to communicate with the subtle bodies in order to find out the message and act accordingly.