Just want to clarify: trousers are singular in Norwegian? In English, the same plural word (trousers) is used whether you're talking about an individual garment or multiple garments. (i.e. 'I'm wearing trousers' and 'He owns both blue and black trousers'). But in Norwegian, if you say 'buksen' you're talking about the individual garment and you say 'buksene' if there are multiple garments. So 'buksene' could never be used to talk about one garment?
Both "buksen", "buksa" and "buksene" can refer to a singular pair of pants - et par bukser.
People with a-ending heavy dialects are likely to stick to the singular "buksa" for a singular pair of pants, but for people who speak dialects favouring e(n)-endings it's not uncommon to use "buksene" in a singular sense.