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

No. This glass lens is designed to replace just the reflective element. Your existing mirror housing and mounting hardware stay in place—you're only swapping the glass.

Will this fit if my mirror housing is cracked or damaged?

This glass lens only addresses the reflective surface. If your mirror housing is compromised, the lens alone won't solve that. Inspect your housing first.

What's the best way to secure the new glass once I remove the old one?

Mounting method isn't specified—test-fit before committing to adhesive or fasteners. Your original housing may have tabs, clips, or a mounting frame; work within what's already there.