Questions about puzzle creation are on topic, so how about asking your own question about the question in question? Then you can add your critiques as an answer. This carries all the normalization advantages you mentioned, while avoiding the spoilers which would occur when discussing the answer within the challenge itself.
One important caveat: make sure your question is specific. "How can we improve this puzzle?" is too broad. Identify only a few specific things which you think could be improved - and which can be done so in a succinctly explained manner - and ask about them.
If you want to post an actual answer to the puzzle, it's OK to do so and include a few remarks about ways the question could be improved. But the actual answer should be the most important part of your post. Answers to challenges which are only critiques are very likely to be deleted as not-an-answer posts.
If you just want to add a minor footnote or aside, then chat or a comment is more appropriate.