According to my opponent, it is generally accepted in Australia that you cannot discard in the charge declaration subphase, because 5.A.2 only mentions declaring or stopping a charge, or passing.

I note that 1.A.6 says you can discard in any subphase involving card play.

Which one applies and why?

1.A.6 is the intention as I understand it - also makes things more interesting with the choice - and I'd go with that given the apparent rule conflict.