Yitay,
I have a longer response to your third point and I don’t have time right now to post it. I will therefore deal with your first two points.
1. The woman loses her kesuba for violating a minhag of B’nos Yisroel (not the non-Jewish culture, as the Meiri explained). Are you disputing that minhagim are binding? (e.g. Yarmulka is a minhag, but the Shulchan Aruch still says a person must not walk 4 amos without one.)
2. With respect to those things that are Daas Yehudis (like covering arms), I agree that it would depend on the norms of the frum Jewish community in which the woman resides.
With respect to No. 3 you are making a mistake in lomdus. I will explain b’ezras hashem after Purim.