Postdoc at a research hospital for a physicist (non radiation)
I'll answer the second two questions.


  Is there anyone who successfully did the physics-medicine transition?


Yes, medical physicists exist.  Some are specially trained in that field, others made the transition.  A few years back they were in high demand.


  And is it possible to go back to more basic science after a
  biochem-med postdoc?


Possible, yes.  Probably few people want to because it's likely to lead to a pay cut, as last I heard there was high demand for medical physicists.