Re: part 2 Oh please! They are CHILDREN. Children do these things. They don't think rationally.
They were old enough to know that stealing is wrong. Most people learn that well before 11 years of age. And if Granger is self-righteous enough to shortly take on the entire wizarding world over house elves, she should have been conscious enough of right and wrong to find some way to get what she wanted other than stealing. It would not have taken much effort. Except, of course, that in RowlingWorld, Gryffindors taking what they want (especially from Slytherins) is considered meet, right and salutary, as the saying goes. And Granger never showed much belief that basic rules, like not cheating or stealing, apply to her personally.