Do I need to remove the mirror housing from my door to swap this lens in?

Depends on your mirror design—some housings allow the lens to pop out without removing the whole assembly, others require full removal. Check your specific mirror housing first; this is the glass only, so you're working within whatever housing you have now.

Will this work if my mirror glass is just cracked, or do I need to replace the whole thing?

If only the glass is damaged and your mirror housing and mounting bracket are solid, this lens is your answer. You're replacing just the reflective piece, keeping everything else intact.

Is this a heated mirror lens?

This is a standard glass lens. If your vehicle originally had a heated mirror, that function lives in the housing and wiring—this lens itself is not heated.