Once again, I have never seen a brand of cat food, dry or wet, which is mostly chometz. Many/most brands I’ve seen do contain chometz (wheat, barley, or oats) but it’s a minor ingredient somewhere down the list. Therefore since cat food is not something normal people eat, and it’s not something one would absent-mindedly eat, it is PERMITTED to own it and derive benefit from it on Pesach. The fact that some exceptional people do eat it is not relevant; the criterion in halocho is שאינו מאכל בני אדם הבריאים.
As far as walking a dog on Shabbos without an eruv one may hold the leash by the end. Whatever is between your hand and the dog is not called carrying, no matter how long it is; only what’s on the other side of your hand is carrying. Scooping itself is not a problem, since it’s removing a public nuisance. But carrying the empty bag in the first place is a proiblem, and putting it in a garbage can is also, because the can is a reshus hayochid. I don’t know how dog owners manage it.