Do I need to remove the entire mirror assembly to swap the glass?

In most cases, just the glass panel comes out — the mirror housing stays bolted to the door. Your service manual will walk you through the release tabs or fasteners specific to your truck's year.

Will this fit if my mirror is cracked or damaged?

This is the glass lens only, so your existing mirror frame and mounting hardware need to be intact. If the housing is bent or the brackets are broken, you'll need a full mirror assembly instead.

What's the difference between the right bottom and right top lens?

They're different shapes and angles — the bottom lens covers the lower half of the mirror and is not interchangeable with the top. Order by position to avoid a mismatch.