[Bigwill]

Pull it is - if the left arm is almost useless in the stroke then I assume we must pull with the right side and arm?

'Almost' is the key word in your post. The left arm is a component of PP4, and so aids in transferring power from the pivot to the club. While actual left arm muscle power may not be very helpful in a good swing, the feel of pulling with the left is something different, and very much a viable option.