Thanks Toritoribe.
So, in (1), would そしたらひっくり返すの手伝ってやっからよ be a correct sentence?
By the way, do you have any idea whether ハリー・ポッターと賢者の石 and ハリー・ポッターと秘密の部屋 are supposed to be at the same reading level? Obviously I'm not asking you to do any time-consuming research, I just wondered if you could find anything about this quickly through Google.