Many thanks, Alex, for pointing out something which I had overlooked --- our ability in MathObjects to control which individual reduction rules are applied or disabled. On the other hand, it is not clear how this might simplify writing of a solution (template) which refers to various parts of an answer expression.
FWIW, the example posted with my query was generalized from a problem with a known relation between $a$ and $b (which was exploited during a subsequent solution). My immediate question involves ways to adapt that as the problem task is generalized.
an aside: my time-to-create a problem+solution seems to average between 1:2 and 1:3 for (problem plus structured answer) : (solution which copes with parameter choices). For those occasions where I am slow in composing an algorithmic presentation of a solution, I am glad for the fall-back of displaying a structured expression as an answer. (Quoted stuff in old style, use Formula or Compute now.)