The Metroid manga's canonicity is a matter of debate. Some aspects might align with the main game lore, but there could be elements that don't fit neatly. It's not a straightforward yes or no answer.
Not really. The Metroid manga doesn't provide a detailed explanation of the Space Pirates. It might touch on them briefly but not in a comprehensive way.
Yes, the final battle with Mother Brain in the original Metroid is a very memorable story moment. Samus has to use all her skills and power - ups she has acquired throughout her journey on Zebes. It's a tough fight and the culmination of the story, as she is trying to stop Mother Brain from using the Metroids for evil purposes.
Quite important, really. The story in the Metroid original manual set the tone for the franchise. It gave us the basic concept of Samus as a lone warrior against the Metroids. This initial story influenced the development of subsequent games in terms of gameplay, as well as the expansion of the overall story arc. For example, it determined the types of enemies we would see, and how Samus would interact with the environment. Without this original story, the Metroid franchise would not have the unique identity it has today.
Sure! A lot of manga publishers translate and release manga in English to reach a wider audience. You can find them in bookstores, online platforms, and libraries.