Yes I agree the demotion should hit hard. Her is hoping she is a mature and strong as Sofia and take the high road and move forward rather tan backwards, although I have a feeling Nick will be very happy as I do not think he like her being his boss.
AS for Sara it is not Greg who brings out her immaturity it is Sara herself. She needs someone who is strong and mature and not wishy washy and for me again that is someone outside the lab.
i don't think she should be demoted. i think they could find a way to make them co-supervisors. just think how happy gris would be to have someone else to officially do the paperwork. besides, cath remaining a supervisor and coming back to night shift would give a whole bunch of drama to the whole sara/gris disaster.