Yitay,
You appear to be assuming that all histaklus is in order to have pleasure. If you learn like that then Rashi is very shver. I am understanding that there are two categories of histaklus. There is histaklus for pleasure and histaklus stam (e.g, looking a woman in the eyes when she is talking to you). According to me kol hamistakel b’etzba k’tanah, with intent to derive pleasure (the ??? ?????? of the Gemara in Shabbos) is assur k’ilu they were mistakel b’makom hatoref, which is assur with histaklus stam.
The Rema in E”H 25:2 does argue on many of the psakim in O”C 240 but he does not argue on halacha that it is assur to gaze at the makom hatoref (which is not brought down in E”H). Similarly, R’Yochanan in Nedarim is not arguing on Reish Lakish, he is arguing on Yochanan ben D’Havai. We pasken m’ikkar hadin like Reish Lakish that it is ???? ?????? ????? ???? even regarding one’s wife. It is assur even if there is no intent to derive pleasure (as pleasure is muttar between a man and his wife).
The way I am learning the Gemara, and as I think is meduyak in Rashi, is not a stirah to the Aruch Hashulchan’s heter. Aruch Hashulchan does learn the Gemara differently than I do, but also differently than you (he paskens ???? ?????? ????? ????).
When deriving “shok b’isha erva,” R’Chisda used a posek that refers to shok as “erva” (as Rashi explains). In contrast when Shmuel says “kol b’isha erva” he quotes a posuk praising the beauty of a woman’s voice and Rashi explains “?????? ?? ??? ???? ??? ???? ???? ???.” Rav Sheshesh then says “sa’ar b’isha erva” from the same type of posuk. Thus according to Rashi we learn out from a posuk that shok IS erva, while we learn out that kol and sa’ar can create tayva. Therefore regarding shok there is issur regardless of whether it actually is m’orer tayva, while kol and sa’ar will only be ossur in societies where it is m’orer tayva.
You are probably aware that there is an issur of davening in front of exposed erva mamesh. As the Aruch Hashulchan says (in OC 74)”???? ????? ?”? ?? ????? ?? ???? ?? ???? ???? ???? ????? ??? ????? ???? ??? ???? ????? ??? ?? ??? ??? ?? ???? ???? ??? ???? ?? ???? ???”
I think that according to Rashi’s understanding of the Gemara, “shok” was given the status of erva mamesh (m’drabbanan) and it is assur to daven in front of, even without histaklus, and assur to be mistakel even without intent to derive pleasure l’gabay arayos.
I am certainly saying a chiddush in the Gemara, but so are you. The Gemara makes no mention of the principle you are reading into it (l’maysa I think I might have answer to my kasha that fits your pshat).
BTW when Rashi says “eshes ish” he means arayos.