07' 3.5 300 leaking coolant behind belt casing, im not sure if its the water pump seal or if it could be something else, also im not 100% about if i change the pump do i have to change the timing belt as well? Plz help thank you

 

I would replace the timing belt and water pump yes

 

Do you know the reason the timing belt has to be replaced even if it looks ok, and does the timing belt tensioner NEED to be replaced, or can you use the existing one?

 

required replacement at 100k you can do what ever you want its your car.