Allowed or rejected

A. Several times I heard from the rabbi that there is no nefm between hotra and dhoya (that is how I remember it) and the rabbi doubted the nefm of the rabbinic nefm regarding the prohibitions. While studying the issue of rejecting the prohibition of a brother's wife because of the mitzvah of yevom, we came across a boundary between letra and dhoya, according to which there is a nefm. I will try to present the things as clearly as possible.
Let's take Yevam as an example: The Torah commanded Yevam to marry a brother's wife, and thus did not obligate us to be subject to the prohibition of marrying a brother (the Tosafot (3:2) saying that this is a law of a deed that is rejected, not to be done). However, at the stage of permission, there are two ways: either against fulfilling the mitzvah, we are not subject to the prohibition – rejected. Or against the act of the mitzvah, we are not subject to the prohibition – permitted (since the law does not contradict the fact that the mitzvah will be fulfilled except by the act of the mitzvah). And in cases such as Katan and Yevam, it is permissible and there is no prohibition of marrying a brother's wife, but the act is an act of the mitzvah, and therefore it will be permitted and there is no prohibition of marrying a brother's wife.