Does this come with the mirror backing plate or just the glass?

Glass lens only. If your backing plate is damaged, you'll need to source that separately or order the complete mirror assembly.

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

No. The lens mounts to the existing housing using factory points. If the backing plate or frame is damaged, the glass won't seal or sit flush. Inspect the housing before ordering.

Do I need adhesive or fasteners to install this?

Installation method depends on your specific mirror design — some use clips, others use a seal. Check your housing setup before starting. Have the old lens out first so you can see what you're working with.