I'm somewhat confused by what people are concerned with. But, if it helps any, here's my take on:
a. Britain - Aquilonia is supposedly more France than Britain (with a Roman element) but has elements of it and the Bossonian Marches are supposed to be Wales-ish, while Brythunia seems almost Britain + Poland = Who Knows?, so there's no obvious answer besides the Bossonian Marches, but there's options.
b. Britain 2 - If you don't care about perfectly trying to simulate Conan's world as written about by Howard and maybe others, it really doesn't matter, just make anything convenient Britain.
c. Official Material - I really don't get why it matters whether some new supplement comes in that details some place that has already been detailed in someone's campaign. If the GM went to the effort of detailing something, then, that's what it is. This is a RPG - GMs are gods, not publishers.
d. European feel - The Hyborian nations are mostly European, though there are all sorts of (supposed) mixes. Some of them seem quite poorly developed, like Corinthia (landlocked Greece?), Ophir (Northern Italy?), Brythunia (Poland?), and Border Kingdoms, to where it's hard to say what they should be and so could be whatever you feel like without giving fits to Conan purists. Giving any of the Hyborian nations more of a European feel than what may come across seems easy enough - call Aquilonia France, Nemedia Germany, Gunderland Netherlands, Bossonian Marches Britain, Cimmeria Ireland, etc. - if it makes for a better world.