Apart from all this tech discussion (which is great) there are some other factors to consider. Namely politics.
No one at the FAA is gonna lift the grounding unless it is absolutely, positively, guaranted fixed so that any monkey could agree the problem is corrected. That includes the President of the US who made (so far as I know) first ever presidential grounding of an aircraft series. I don't see any bureaucrat in either the government or Boeing putting their reputation and company survival on the line until all possible questions have been satisfactorily answered. Oh and don't forget the lawsuits and congressional investigations which are just getting started.