About the prohibition for a non-priest to publicly pronounce the priestly blessings

The Talmud in Ketuvot 24B spells out clearly that a non-priest is prohibited from publicly pronouncing the blessing and the Talmud calls it an "Issur Aseh" (a negative commandment derived from a positive one). Rashi explains that the positive commandment reffered to here is "Koh Sivarchu" (So you shall Bless). It is therefore extremely perplexing that Tosafot in Shabbat 118B says in the name of R"i that "R'i did not know of any source to prohibit a non-priest from reciting the priestly blessing in public if not for the issue of his making the initial blessing in vain".

The Rama in his work "Darchei Moshe" says, cryptically, that there is a difference between when he recites the blessing with other priests as opposed to his being the only so-called priest reciting it for the public. Obviously this needs some explaining as there is seemingly no halachic difference between the two situations.

The Hafla'ah gives a brilliant explanation to this seemingly cryptic Darchei Moshe. He says, that there is one halachic contingency for the priestly blessing to be considered valid. That contingency is that the priest has to be called to bless the public by the public. This is the reason the Shliach Tzibur cries out "Kohanim" before he starts the blessing. Based on that Halacha, The Hafla'ah says we can understand very simply the difference between the Darchei Moshe's two cases. When this non-priest is among other real priests being called upon to bless the congregation, he is then violating the commandment of  "Koh Sivarchu" as he is impersonating a priest. However, when this non-priest is the only reciter of the blessing we can say that the "Shliach Tzibur" never intended to call him up to bless the congregation, he therefore is not considered to be blessing the congregation nor is he doing anything prohibited as Tosafot states in Shabbat.

The Chasam Sofer interestingly has a direct opposite take on this Darchei Moshe than his Rebbe the Hafla'ah. He says that when there are other priests together with this non-priest, then we can assume that the Shliach Tzibur wanted only to invite the real priests and not the imposters and therefore when other real priests are present this not priest is not considered as being called up or invited and not prohibited. However, when he is the only one, we cannot say that the Shliach Tzibur did not mean to call him up because he is the only one here and therefore is definitely considered a transgressor of "Koh Sivarchu"