I think so. When typing checking a project, its dependencies don't get type checked as well as far as I know. Type errors in dependencies are kind of not relevant for you, and many dependencies are not type-annotated anyway.
Well you probably do still want the type checker to at least look at the public interfaces of any libraries you depend on, since you want it to enforce that you're calling into them correctly.
Absolutely. It should probably not report errors that are not in your code, by default anyway, but still extract information about typing relevant to you as much as it can.