Wow, what a strange debate (haven't read it all i'll admit) I'll just add I did say it is likely a college would initially ask a student underage to defer, not that they would demand they would or not give them an offer.
Also, why do you think you need to be in Cambridge doing exam syllabus to do maths? Is all other form of intellectual pursuit "twiddling your thumbs"?...(is it your kid?) they can train up til they get 42/42 on IMO, should occupy anyone.