CONGRATULATIONS!
Vigilence is the only thing I can recommend on the catbox issue. (I have a 10-year-old Yorkie who likes to raid the 1-year-old cat's box on occasion.) It's in the bathroom so we are able to close the door most of the time.
Definitely get a vet check. And we have always used puppy shampoo on Ozzie because his skin is sensitive. I share my Pantene conditioner with him and he doesn't tangle as bad as with doggy conditioner.
It's very good that you are both switching her food and doing it slowly. Ol' Roy is definitely not the best food.