So, is drzwi plural in form, but singular in meaning? This -ami ending would generally be plural, wouldn't it?
Actually, the same. Even one 'door' is already plural (pluralia tantum) in Polish, so adding the second door doesn't change anything.
The drop-down translation (at least on the web version) says that 'drzwiami' is locative. According to Wiktionary, however, the locative is 'drzwiach.' Am I missing something?
No, the hints were indeed wrong. Fixed now (may take some time to implement).
It is a good habit to double-check the declension on Wiktionary, especially that the hints do not necessarily apply to the sentence you see at the moment, but to the word itself. Or sometimes there's just a mistake.