Does "someone appeared to help me" have two meanings?
You are right. The only thing I'd like to add is that, in case it means "someone seems to be helping me", you'd use the continuous form "be helping" in most contexts, not the simple infinitive "help", because it is about someone who is busy helping you now.
Yes. This is called lexical ambiguity.
In related news, "Time flies like an arrow" has seven possible meanings.