It’s not a law of maths in the way that 1+1=2 is a law

Yes it is, literally! The Distributive Law, and Terms. Also 1+1=2 isn't a Law, but a definition.

So 1/2x is universally interpreted as 1/(2x)

Correct, Terms - ab=(axb).

people doing academic research in maths, not primary school teachers

Don't ask either - this is actually taught in Year 7.

if they realise it’s over a question like this they’ll probably end up saying “it’s deliberately ambiguous in an attempt to start arguments”

The university people, who've forgotten the rules of Maths, certainly say that, but I doubt Primary School teachers would say that - they teach the first stage of order of operations, without coefficients, then high school teachers teach how to do brackets with coefficients (The Distributive Law).