Sam2 – I haven’t really gotten involved in this discussion until now, but having gone through the Rashba you brought I took it to be saying exactly the opposite of how you learned it. The last line (which I presume is the crux of your pshat) is talking about yochid (or small group) haragil b’issur, in which case his (or their) continued issurim cannot turn it into a heter for the rabbim so that ein choteh niskar. Now presumably he’s referring to Jews here (otherwise it’s not an issur). The rest of the teshuva says quite explicitly many times that the issurim of beged ish/beged isha is toleh on mokom AND z’man, in other words when the mokom in general at that time does not view something as beged ish/isha it is not. To me trousers (as they are correctly called in English… :-p) are an example of the latter, the world in general across most continents for most of history has seen them as gender-neutral clothes. For a short while in the 17th-19th Centuries in Northern Europe they were seen as predominantly (but not exclusively) beged ish, in which case then and there they would probably have been ossur for women. But seeing as the world in general (especially the places where most Jews tend to live..) nowadays has women wearing trousers, long before any yechidim or small groups within klal yisroel did so, this would not be a case of choteh niskar, as the mokom/zman already dictated that it is not specifically beged ish.
Sorry that was a bit long-winded, hope you understand my understanding of the Rashba…