tredmill or the elliptical

it doesn't matter, you burn roughly 10 calories a minute on either of them. whichever one you will use more and push yourself harder on is the one that will be more effective for you.

if you have bad knees, you would want to probably start on the elliptical because its an easier movement.

Either. It really depends on the intensity -- grab dumbells/medicine ball + incline. My fave is the elliptical, it is more controlled for me e.g. I squat down when I want a good lower body burn, then come up and focus on arms. But if I want to really work out my lower body I get on the treatmill because you can do some drills, run sideways, also I put it in low speed while incline is max and get some lunges in. I alternate between different machines because of the variety.

You can really boost your cardio workouts in many different ways -- so it really comes down to preference IMO