R. Chanina MiSura tells us not to do kinui (warn) a wife today because perhaps the halacha is like R. Rabbi Yosi beRibi Yehuda that kinui can be performed even without witnesses and then a woman could be assurah to here husband without the availability of the mayim.
Why do you think that R. Chanina MiSura told us such a thing davka with regard to Soita. I can't recall a similar statement in other places. (I will try to do a computer check of dilma kayma lan) why davka here is he choishesh to a particular opinion. In fact, the halacha is like R. Yehoshua and not like Rabbi Yosi beRibi Yehuda. the issur of a husband to have relations with his wife who is a soitah is not an issur misa or like an issur eishes ish where we are very machmir. i know that you can answer that in such a case the issur is continuing etc., but i am looking for a more conceptual answer.
dmartin, raanana, eretz yisroel
Since there is absolutely nothing to be gained by making Kinuy, and there is no recourse if she does become Asur, it seems sensible to be Choshesh.
Obviously, if someone did do Kinuy, we would be Matir her.
Re- the comment of R. Chanina mi Sura about kinui bizeman hazeh beino leveinah:
The Shulchan Aruch paskens not like Rabbi Yossi beRabbi Yehuda and still paskens like R. Chanina, and then it is mashma at first glance from the Rama that in fact we would not matir her (as you say). I heard that there is a pischei teshuva on the issue, which I will also try to review soon.
The Rema is not arguing on the Shulchan Aruch and the Tur. He is merely continuing the theme of the Chashash of Rebbi Yosi b'Rebbi Yehudah -- that if she is Asurah, there is no way to be Matir her. But if he does [do Kinuy] we will not force him to give a Get as the Beis Yosef and Bach rule (see also Pischei Teshuvah).