These threads are always so odd. I don't know anyone applying for Eton or Marlborough or Wellington, but around here, there are plenty of private schools that are very highly rated and all have various tests and assessments that children have to do in order to get in. Some have higher standards than others because they're more academically focused. Others have more interviews etc. Depends on the school.
My thinking is based purely on my own perceptions from reading the press etc but my view is that Kate would divorce William before she sent George to Eton. I agree that a co-ed would be something I think they'd be interested in to keep the children together. I also would have thought that a slightly more flexible boarding option would be preferred but I do see why boarding works - a lot less security risk and concerns in terms of going to and from school daily.