That there is a basic maxim doesn't mean there isn't a) an explanation of why that maxim is of overriding importance and b)an explanation of how that maxim leads to particular actions

Presumably meaning that it isn't obvious how you get to (a) and (b). Phils. are very aware that you need to get to (a) and (b) and have argued elaborately (see Kant) towards them. (Has anyone here read so much as one wiki or SEP page on the subject?)

Quite. In order to have a good deontological basis of ethics, both (a) and (b) are necessary; and I would expect to find both. These build on and enhance the maxim on which they are based; indeed, these would seem, to me, to be the two things that change a simple maxim into a full deontological basis for ethics.