Just curious: are you sure that everyone/the vast majority of the players on the server are Japanese? Perhaps I’m spouting nonsense, but this sentence feels wrong to me. I’m pretty sure it should be 「…を＊読む＊ことが…」.
Don’t think it is intrinsically rude, but you responded to something written in 丁寧語 (polite language) with a sentence in タメ口 (casual language). I don’t think that’s appropriate. I could be wrong.
Yes, probably, because it’s both explanatory and adds emphasis. のだから, for example, can come across as insistent, and is often used to call the listener’s attention to a difference in opinion/awareness between the speaker and the listener, particularly when it seems the listener hasn’t grasped the importance or implications of something. Perhaps more importantly, there was no reason to add emphasis or an explanatory nuance following ‘I’m/it’s not absolutely fluent’. That’s why it seems defensive.
I think it’s just rarer. I doubt that it’s wrong. Perhaps the other reason 勉強する makes more sense is that one of its other meanings (which is closer to how it’s used in Mandarin) is ‘to make an effort’. If you said that you’re not yet fluent, it makes more sense to use an expression that suggests you’re making an effort to become fluent, as opposed to simply studying and acquiring knowledge.
Yes, it’s true that this doesn’t make much sense, even if the meaning could be guessed based on context, provided you know what a Japanese learner’s priorities might be.
You can’t drop an element that you haven’t mentioned earlier in the conversation. I personally thought this sentence might have meant ‘among other reasons, [I’m learning it] for this game’, but it seems I was wrong. If you wanted to say that you’re learning it in order to play games, including the one that you were playing, I think you could have said 「ゲームのために日本語を勉強しています。このゲームのためにも。」(‘I’m studying Japanese for games. For this game too.’)
EDIT: If you want a version with 中, then try this as a substitute for my second sentence: 「その中に、このゲームも」 (Any one of the following phrases is implied after も: あります OR 含まれて・入っています. They all imply that ‘this game’ is present/included among the games you’ve just mentioned.)
The problem with と言えば is that と what comes right before it, which would mean you’re referring to the previous sentence, which is irrelevant to what you say next. そういえば would probably work, in my opinion, because I’ve seen it used to mean ‘speaking of which’ in a fashion that suggests one has just remembered something.