One witness on the matter of allowing a woman to remarry due to the death of her husband

This is not my invention. This is the method of all the poskim and this is how it is explained in the Gemara (although the Rambam has a contradiction on the matter, and it depends on various reasons in the Talmud itself: a definite presumption and a minasva or the regulation of agunot). The explanation for this is very simple. If the husband is in fact dead, the woman is permitted to marry. Although there is a rule in the laws of evidence that only two witnesses permit a private matter, but this is only the laws of evidence. Therefore, if the Sages are convinced that the husband is truly dead according to one witness (and a definite presumption and a minasva), they can relax the laws of evidence and allow marriage. They did not allow a man's wife to be an Alma, but only relaxed the laws of evidence. In principle, sages can remove something from the Torah in a Sho'at, and when there is a great need, then probably also in a Ko'a. If the permission is to marry, then it is a permission in a Ko'a. But if the permission is not to accept the testimony of two, then there is room to discuss whether it is a Sho'at or a Ko'a.