datta kana, mite yaru

1) An old man is reciting a local legend. He gives his usual spiel, then ends the story with "...じゃったかな。" I know what "jatta"/"datta" and "kana" mean independently, but I'm a bit confused as to what the comment means. "I wonder if that was it" (meaning, "I wonder if that's how the story goes")? "I think that was it" (meaning, "I think that's how the story went")?

2) A group of teenagers is trying to get a rough older girl to give them an item they need for a project at school. After an argument, she agrees, but she insists on coming along and watching them while they borrow it, saying, お前のやったことの結末はみてやる! I understand this sentence means something to the effect of "I'm going to see the results of what you've done!", but I'm wondering why the "yaru" is there. Her coming along is a condition of borrowing the item, not a favor to those she's addressing, so I don't know why "yaru" is used.

Thank you for any help.

1. You are correct. It means "I think that's how the story went" or "If I remember correctly", etc.
2. みてやる=みてあげる,if she was not a nasty girl meaning I shall look at(and comment/judge/criticize) the result (after seeing how the borrowed item is used).

1) Yes, you've got it. I can just picture an old man telling the story, and nodding to himself, saying "... or so I believe that's how it went" or something like that.

2) "(te-form of verb)+ yaru" is the colloquial version of "(te-form of verb) + ageru" so grammatically it might sound like she's doing them a favor, but in this colloquial, rough talk, it's more like "I'm going to do that juuuuuust for you, alright?" or "You'd be better be prepared for what I'm going to do!" sort of sentiment.

Thank you both very much.