I think such "answer comments" are par for the course on this site, precisely for the reason you gave: the comments are actually "asking questions requesting clarification on a design". The questions are just posed in the form of "would X work in this case?"
I'd go as far as to say that questions which receive that sort of comment and then go unreplied by the OP should perhaps be closed as "Unclear", since we as a community don't understand the OP's situation well enough to give a valuable answer.
Keeping such responses as comments has two main advantages in my view:
"Answer comments" aggregate all information under the question
I much prefer having these in comments than answers precisely because the user might then have to comment on the answers saying "sorry, but actually that won't work in this case because of Y".
The OP will then have to either remember or be reminded to edit their question with that clarification. And given the volume of new users we have here, they frequently "forget" to do so, in which case future potential answerers then have to read through the comments under both the question and all other answers to get the full picture of the OP's situation.
By having such things as simple comments under the question, everything is more or less tidy in one place, easy for everyone to see.
"Answer comments" are likely low-quality answers
Putting such things as answers also has the problem that is doesn't fit Glen's summary of what the site is looking for in answers:
We (the site / the community) are looking for high quality answers that explain Why and not just a simple What or How. One or two sentence answers generally don't do justice to answering Why.
Answers which have to be couched in terms of "X might work in this case" (precisely because the answerer doesn't entirely understand the OP's situation) will have a hard time stating Why, just What.
Answerers may therefore feel disincentivized from answering or commenting at all precisely because such answers would be relatively low-quality. Especially since the higher standard for answers make users feel the need to invest more time on them, time which risks being completely wasted if the OP comments "that won't work in my case because X".
Users are far more comfortable merely asking "would Y work?" and waiting for a reply from the OP. If the OP agrees, then the answerer can flesh out a high-quality answer which not only states that Y works in this case, but also Why it works.
Now, there may be some instances where a user sees a question and comes up with a potential solution, but doesn't feel like they have a full grasp of the OP's needs. However, it seems hard to fathom (or very unlikely) that their solution won't be satisfactory.
In that case, I'd certainly recommend skipping the "would X work" comment and moving straight to an answer. However, the point at which the user is willing to risk the time investment is subjective and not something I'd be comfortable determining.