1) I don’t know what R’ Henkin ZT”L held, but the halachic norm is only to allow 18 tefachim, as per R’ Moshe ZT”L, and only b’dieved.
2) The issur is not only to look. The reason is also to avoid “kalus rosh”, hence, R’ Moshe holds that the ordinary 10 tefach shiur is not applicable here, and in fact, is meikil (b’dieved) even if the mechitza is transparent.
3) R’ Moshe writes (and proves) the obligation to have a mechitza is d’Oraisa. (O.C. 1, 39)
4) R’ Moshe is referring to the issur of having an improper mechitza. I didn’t see him address the specific issue of someone davening there if its already that way. However, he writes that one is obligated to protest with all his might against a mechitzah which is below 18 tefachim, so it’s mistaver that it’s assur to daven there (even at the expense of t’filah b’tzibur), and, in fact, you’re probably obligated to register your protest at the disregard for halachah (although I don’t know if it’s a situation in which it will definitely fall on deaf ears).