"Dernières" (before the noun) used in this context undoubtedly refers to "the latest" as in style/fashion. "Du dernier chic" = extremely chic. Other uses include (when it is placed before the noun), "final". Many other uses listed here as well: http://www.larousse.com/en/dictionaries/french-english/dernier/23949
The problem I have is deciding what DL will accept. I assumed the sentence meant 'latest skirts' but didn't want to risk losing a heart so went with the safe literal translation which is usually accepted. Thank you to everyone who contributed in the discussion and clarified the meaning.
Try not to think of it as rules, but, like you also said: patterns. It's a pronunciation thing. The pronunciation of "derniers" is on par with "dernié" whereas adding the "-e/-es" onto the end makes you have to pronounce that "r," so the word shifts to "dernière(s)."
Accents are there (in most languages) to help guide your pronunciation, so they most all logically follow from however you'd be pronouncing the word. I've seen the same with German Umlaute and with Spanish accents.
It's very much a pattern of pronunciation.
Dernière only means "top" in the sense of a space far away when considered vertically, e.g. the top drawer of a dresser or top floor of a building. Even in that sense, it's still a sort of "last" or "final" when starting from the bottom of something. It's difficult to see a case for that here. I suppose you argue that it refers to some dresses on the top rack or something, but that's a real stretch.
BANGS is a rule of thumb, not an absolute rule. When an adjective has a figurative meaning it goes before the noun, when it has a literal meaning it goes after the noun.
Numbers always go before the noun. "dernier|ère" (last) is the opposite of "premier|ère" (first). In this sentence, "dernières" means "latest" or "last so far" rather than just the usual meaning of "last", because it's talking about the latest fashion, not the last dresses.