If they're not on, hdds can handle surprising amounts of acceleration, like 100s of gs

Yeah. Fluid dynamic bearings and parked heads don't seem very vulnerable to shock and vibration.

You could almost invert your logic too. Perhaps the spinning on the HD throws off the telescope enough to be troublesome. I'm not sure how stable the lagrange point orbiting is, but it can't be super stable.

Any moving mechanism is potentially a source for disturbance to the telescope. Not something that affects the stability of the orbit. But the small vibrations can translate to small vibrations in the instruments and secondary mirror which then cause distortion over the integration time of the image.

I don't know that that is the primary reason HDD's have been avoided. But any moving part is another source of failure so my guess is the HDD's life is not as long.

As far as I'm aware I don't know of any spacecraft that has flown a HDD (but there certainly could be). However, some early spacecraft did use tape drives. Hubble originally used tape drives and was replaced with solid state memory during one of the servicing missions.