My experience with refactoring is if a change causes larger number of tests faults, I run the last failed test and fix that (see my AGENTS.md posted elsewhere). Generally, if you fix the one issue everything else falls in line.

Wrt mocking. I'm not a huge fan. Again, look at my AGENTS.md. I prefer monkeypatch as a last resort option. Luckily, if you use TDD, you rarely have to use mocking. If you don't use TDD...