“You said, their body shields the hadaya vatthu. How dense must their body be then? If it would be a few rupa only, would that be enough to shield?”
We cannot think about “shielding” in terms of our perceptions.
– The human body “shields” the gandhabba (hadaya vatthu plus pasada rupa) inside.
– The human body is not concrete or steel.
– We have no idea how kamma works. Only a Buddha would know.
See, “Brain – Interface between Mind and Body” and “Clarification of “Mental Body” and “Physical Body” – Different Types of ‘Kāya’“