Does a divorce, uttered with the intention that it not take effect, and during a period of purity in which the husband had intercourse with his wife, nevertheless take effect, especially since the one who uttered it was unaware that the majority of scholars consider it to take effect despite its contravention of the Sunnah?

The relied-upon fatwa is that a divorce pronounced during menstruation or during a period of purity (tahr) that was preceded by intercourse, is a valid divorce despite its being bid'i (innovative/unlawful). This is the view of the majority of scholars. Since you uttered an explicit divorce, it has taken effect even if you did not intend it, because an explicit divorce takes effect without the need for intention. If this pronouncement does not complete three divorces, you can take your wife back before her 'iddah (waiting period) ends.