If you take the time to read through the website given to you earlier it has some tips on how to get kibble addicts to eat healthy, wet food.
Lots of misinformation out there as to what is good, what isn't. Unfortunately when you have the very people who are supposed to be looking after your cats' health telling you that a diet of only canned food is bad because it doesn't look after their teeth it sure doesn't help, does it? Truth be told the dry food causes more issues with teeth than does the can. If you really want to keep a cat's teeth clean you would either brush them yourself (sometimes a battle until you get kitty used to it) or you can put down a
raw chicken neck for kitty to munch on. Nothing better to clean the teeth than something they can sink their teeth into and really have to work at chewing it.
Here's a nice, short article that explains a few things correctly -
http://www.cathealth.com/food-wet-dry.htm
I think a few of the things that desperately need to change in your home are:
Less stress - less yelling, NO corporal punishment for the cats' mistakes. It could very will be that this punishment has made the issue as bad as it is now, not better. Cats are like little sponges. They pick up quickly on vibes in a household.
Ditch the dry - canned only. Even if it is only the FF pate style foods. Canned food can be left down all day long with no ill effects. Yes, you may have to poke around and find something your cats both like. Yes, it may take a while. But it is critical for both your cats' health.
Diet - please do not rush on expecting your cat to lose weight. It took her a long time to get to this point. It will take longer to lose it. You can look at it this way - equate 1 pound of cat weight to 20 pounds of human weight. Compare how long it would take a human to lose that 20 pounds on a sensible diet. That should be the amount of time it will take your kitty to lose 1 pound. But please, at the same time, do not starve your cat! That can lead to fatty liver disease which, if not caught and treated fast enough, can kill your cat.
Try spending 10 to 15 minutes a few times a day playing with your kitty. Maybe dangling a want type toy above her head so she has to make some effort to get it or walking around with a string trailing behind you. That's a popular one in my home.
Please note - talking to your vet about your cat's diet when your vet has already told you that dry food is what cat's should be eating will do your cat no good. The carbs in the dry food is what made the cat fat in the first place. She simply will not lose weight eating dry food!
If you can work on that (the underlying issues) you may well find that the litter box issues will solve themselves. Although, knowing that the box is in a very noisy garage with lots of loud noises, I still feel that one of those loud noises just may have thrown her off using the box to defecate.