...AND is it a breach of etiquette? I do not think anyone has done so but I, for one, often thought of answering my own question in negative given the option is permitted. Will this be frowned upon?
It is not against general SE policy to answer your own questions. In fact, it is explicitly encouraged. The main idea is not, however, that this provides a good opportunity to boost your reputation (I suspect users would catch on and would not continue to upvote you if that was the sole purpose), and nor is it the idea to pose 'puzzles' (this is never explicitly stated, but I do not think that is okay; this has been brought up on MathSE and occasional puzzles are tolerated, but unpopular).
The purpose of this site is for people to be able to get answers to problems that they face in one form or another. So if you ask a real, answerable question, and later give a good answer, then that can only be good!
That setup you describe seems to be a legitimate and recognized rhetorical pattern of setting up a premise to be knocked down (either a straw man or reductio ad absurdam, or simply a reversal).
It might seem a little disingenuous or deliberate or mercurial, and on the surface just strange, but it is not in and of itself a breach of etiquette. No one will be inconvenienced or hurt by such usage, only possibly confused.