(This series is called "Destroy this text" since I'd love to receive sorely harsh criticism on my texts in order to improve, no matter if we're talking about the pettiest mistake, I need to write a perfect English).
Well, I have settled an objective and now I'm supposed to write whatever comes to my mind for the next 30 minutes because I really need to improve my writing in all of the languages I know. I've taken the decision that I won't look up for any word, for any grammar structure or spelling. This will be hard, but I hope it will be fructiferous. This globalised word necessitates an wondrous writing ability, however, if no one corrects my mistakes, I will not be able to improve and will keep making the same mistakes. I consider the written capacity as a demonstration of one's personality.
In my mother tongue, Spanish, I can tell if somebody pays attention to the text he or she writes, I can see if this person cares about the grammar, the structure, the syntaxis and coherence in his/her text. I highly appreciate a well-written essay or document, and, being an orthography and grammar freak, I gladly know there are more people like me in basically every written language of earth. Possibly, some day I will be a scholar and, in light of English being the globalising language "par excellence" [ATTENTION: I DON'T KNOW IF THIS EXPRESSION EXISTS], and academia being a mostly-written thing, it's perentory to [IS IT "TO ME" OR "FOR ME"?!] me to improve this ability as soon as possible and narrow my errors down as much as possible. I have to carry out and investigation on how punctuation works in English since it's entirely different from language to language: hyphens, commas and dots won't be used the same way in any language. I find this especially hard due to the few and completely specific rules. In Spanish, we have the Real Academia Española, and this institutions provides certain guidance and reference to both native speakers and Spanish learners. Furthermore, I need to employ more formal words as well as idioms and phrasal verbs, since they are used right, left and centre in the English language, no matter if it's formal or informal. In addition, according to myself, there are different academies or institutions which provide regulation to the English language and they vary A LOT, which means that it won't be the same if you decide to follow, for example, Cambridge rules compared to Oxford rules. I am of the opinion that there should be a unified regulation and I'd champion for it. Providing another example, Portuguese has different spellings, but there is an agreement on this, so technically there's an "instructions manual" that helps to elucidate most of the orthographic doubts an individual could have.
Great job! I hope my nit-picking is helpful. As to your point that there are many different rule sets for English, I think you might be confusing style guides with grammar rules. As far as I'm aware grammar in English is fairly consistent, but stylistic choices and spelling vary from one area to the next. British and American English have different styles, and as an editor in the US, I switch between Chicago style for books and AP style for newspapers. The Oxford comma, or series comma, is the subject of an infamous style debate, but whether you're using it or not—it's all grammatically correct. I think this is why editors still have jobs lol. So which style guide you choose to use will depend on your audience, your preferences, and your location. I don't know if that helps or makes the situation more confusing, but c'est la vie. :P