1. Most likely the older animators were getting tired of McKimson's stifling direction. Reading Mike Barrier's interview with McKimson, it's clear he had no affinity for Rod Scribner, in particular. Sadly, his layouts, while professional-looking, just weren't the most expressive. (Take a
look at how Scribner improved on this layout from "Fool Coverage".)
2. Jones's early cartoons, being Disney imitations, clearly tried to be as detailed as possible, to the point where at times the characters even looked "shiny". The round designs, if nothing else, are at least solid drawing.
3. Cats make good material for comedy. They can be sly or stupid or just instinct-following, but they're also domesticated and common.
And whether or not cats being beaten is funny depends on the execution. I'll be darned if "Chow Hound" and "Feline Frame-Up" aren't funny.