The first one is for "simply you are looking for someone who can help you". The second one is for "you are asking wether there is anyone who can help you" and in my opinión, in Japan, it sounds like "someone should help me" depending on the situation. The meanings are similar but I think the nuance is different in Japanese. If I look for someone to help me out in italki to study language(s), I wouldn't use the second one. I would choose the first one but I'll change a bit: おしえてくれませんか or てつだってくれませんか, for example, 日本語をおしえてくれませんか / 日本語をべんきょうしています。てつだってくれませんか。 I hope this was helpful.

"だれかてつだってくれませんか"means "Could anybody help me?" "だれがわたしをてつだってくれますか"means"Who is a person that help me?" I think second one sounds little weird and like you're sure that there is someone who help you. If I need someone to help me,I usually said "だれか手伝ってくれませんか".

Same meaning. There is no difference between two sentences. But second one is more natural I think.