I'll try my best to help but don't take my word for it.
For 1) it seems like you got the gist of it, but for me when I hear "いいか" it seems more like, "Is it (really) OK?"
2) How comes lots of normal Japanese words are written in katakana instead of hiragana in manga? It's not like words that are loanwords in Japanese. I mean natural Japanese words, like: eg. だめ -> ダメ?
It sort of serves as italics, to kind of stress the word, or so I hear.
3)3)Can someone please explain なんで and なんでも? Especially なんでも, I think I understand that なんで is a colloquial way of saying why (as opposed to なぜ). なんでも confuses me though.
なんでも can be "anything" or "everything" i.e. なんでもいいです "anything is fine" or なんでも食べます "I'll eat anything".
4)What does ぶってくれ mean? I kept on seeing it earlier in the manga, and I figured out that it meant "wave" (as in greeting with the hand), but it was driving me mad because I couldn't find it in Denshi Jisho.
I'm pretty sure that is the て form of 打つ(ぶつ) which means "to hit" or "to strike." ふる is "to wave".
5)What about using くん and くんな in place of くれ? From my understanding, it seems like slang. Can anyone give me any general idea of the differences in usage among these three terms?
I have no idea, sorry :sorry:
6) I see people using the kanji 手 to refer to themselves (I think) in sentences. Are they using it to refer to themselves? Is this some slang way of doing it? Some example sentences of it being used this way are:
とーちゃん手が話せないからな俺の分もよつばがぶってくれ 。
and
手がぶるえる。
and
どっから手をつけるかな。
I've never heard of it used that way, but it seems like something might not have been transcribed properly. For example, 手を放す(てをはなす) means to let go of someone's hand/arm, so maybe that's what was meant in the sentence. 手が話せない(hand can't speak?)=>手が放せない(can't let go) So it might be like, "Because Toochan couldn't let go I was also hit by Yotsuba," but that's just my guess.
I think the same applies to 手がぶるえる. There is a verb that is very close to ぶるえる (I'm not even sure that is a word) which is ふるえる (furueru, as opposed to burueru) which means "to tremble" which makes more sense here i.e. trembling hand.
However, I'm hardly an expert so you should probably listen to the more seasoned member's opinions first.