All of the competitions since the first season have been invitationals, not open competitions. A few studios are invited and they only offer the categories that Abby has dancers competing in. Sometimes there may only be a few competitors, like maybe only 2 trios, so the girls on the show will come in either first or second. They could let every girl have a sol every week, but that wouldn't make for a very interesting. They film the dances more than once, but that doesn't mean they are scored more than once. They also do things like edit the show to make it seem like 2 girls were in the same age group when really they weren't competing against each other at all.
As far as the fight last week, I don't think Kelly should have made contact at all, but it would be hard not to at least poke someone who was lunging at you like that when you're pointing. It was cropped pretty tight, I wonder if Abby's lower body bumped her first. I still don't think it's ok to pull hair like that, but I can see the situation escalating if you are bumped and feel threatened by that.
In the end, for me, a lot of the over the top things Abby and Cathy say or do cross a line, but the mothers put their kids in those situations for the sake of getting their kids on TV. Yes, they have second thoughts and are stuck in contracts they're not happy with, but they thought it would help their kids' careers to be on the show, and these are the storylines, and I'm sure they realize that if they just smile or shrug at Abby's rankings and comments, their kids might be off the show and the benefits that come with it.