Why does "has retired" work?
It means the same thing.
우리 should be interchangeable with both "My" and "Our".
In other threads, someone said to mean "our", 우리들 is more likely to be used to avoid confusion.
What's the 께서 for?
It is a suffix used for honorific. You can see the explanation in Honorific Lesson
Why 'My grandfather has retired.' not accepted?