Hi Mover123, if one thinks about some of the things you describe this could be associated with a low back issue of some type. It is possible (and probable) that a dancer can have what is called a spondylolithesis in their low back. The can cause instability and at times can lead to pressure on a nerve etc. Nothing to worry about though as it is often said you need one or two levels of these to have the flexibility needed to be competitive.

A standing quads stretch is also a loaded lumbar spine extension hence you are not just relaxing the hamstrings. You are at the same time stretching the psoas and femoral nerve. Either, or both, could be involved therefore in the symptom behaviour. The rotation forward bend is also a rotation and side flexion movement on the low back which causes compression as well. And your fiugure 4 description also involves the back, pelvis and other neural structures via their muscle associations.

The best thing to do here would be to try to reproduce the symptoms with a PT watching. They can then make slight adjustments to see how that affects them. It might be that an alternation to the spinal position reduces the symptoms which would lend strength to the notion of the spine being part of the issue.

Do let us know how you get on. Oh and BTW, an X-ray would tell you if you have a spondylolithesis or not (more as a positive result as a negative might just mean that it's not obvious in standing in neutral).