Upon further investigation, I answered my own question as follows: in the problem in question, we have
and the further down in the same problem:
So any updates done to the "Vector" context (including the reduction->set in the original problem text) apparently have no effect for the later typesetting of the implicit planes. However, if I put
*after* the Context("ImplicitPlane"), then the undesirable formatting can indeed be turned off. The reason my earlier efforts had no effect is that I put the "noreduce" command after Context("Vector") near the top of the problem.
For this problem, I suggest adding
Thanks, -- Peter