The UK tried to define their standard “Received Pronunciation” (RP), and that’s what you’ll find as the “British” pronunciation in dictionaries. There was a backlash, and the BBC no longer uses RP exclusively. In reality, “British” pronunciation varies wildly from one London neighborhood to the next, and accents in other parts of England are more foreign than US or Australian accents. And then there are the other UK countries: Wales, Scotland, Northern Ireland. Australian accents also vary a lot, but the “target” is again the RP. American accents (including Canadian) are mainly based on that of Southwestern England (from a few hundred years ago), along with influence from Irish English and some other UK accents. American (including Canadian) accents are very harmonious compared to those of the “old world”. Learn whatever accent you are most likely to come in contact with, or to find out the number of native speakers from different countries, see the Wikipedia article, “List of countries by English-speaking population” for more information.

No, it's not possible. Spelling in English doesn't make sense. Yes, the history of a word has much to do with the way it is spelled. For instance, words that came into English from German often retained their German spelling even though the sound changed. Many of the words that start with a silent "k" are like that. They come from German words where the "k" was pronounced.

Hi, most native English speakers understand other native speakers. We all have different dialects, and even regions within countries have specific slang and dialogues. To ensure you can understand as much English as possible, I would work with people from different English speaking countries so you can ensure you can understand the differences in speaking, pronunciation as well as regional word nuances. It will also help you understand non-native speakers better as well.

I would just pick which country appeals to you. Not even native English speakers know or can understand other countries that have native English teachers. I've been to Ireland, Scotland, Manchester and London they have different words, accents and slang. I've spent a lot of time around Aussies same thing what the..... are they talking about lol. However, if you listen carefully or ask what that means you will be just fine. So whichever country or accent is appealing to you study with a teacher from there.