Hello retrorobot, and welcome to the thread.
On point 1, I'd agree that 50% means a lot less than 50% in practice, (although don't forget that many of the siblings taking priority will be siblings of children who took open places).
My local CE primary, which has 60 places, operates 30% open admissions, implying 18 open places. In practice the number of non-sibling open places is usually in single figures. It was 6 in 2007 (the only year I know the numbers for), and they were all within 200m of the school.
The reason people were pushing for 50% locally for St RR is because that is the maximum allowed for new Academies and Free schools. St RR wanted (and got) close to 100%, and it was able to do that because it was (controversially) set up as a VA school instead.
There are many people who would have preferred 0% foundation places, but 50% seemed to be a reasonable compromise to argue for, given the legislation that was in place.
On point 2, that's interesting and I didn't know that. I can see how it might raise eyebrows, but as well as the independent-spouse defence, don't forget that she will be one of several governors of the school, so may well have argued for fully open admissions. If she made that case, and lost the argument, should she resign? Should she issue a statement to separate herself from the decision? I don't think so. School Governors need to represent a united front externally, but moderating voices within them should stay on board rather than jump ship.