In terms of obligations: An outside boat is not obligated to allow an inside boat to sail her off the course. Nothing said here by you or Matt has convinced me otherwise.
I will agree that unless L luffs to force the tack (and keeps clear, etc.), they can sail away and W isn't "obligated to tack." But, L is not obligated to allow this to happen once the point of reasonable mark room has passed.
Mike
EDIT: In my head, this is simple. Thanks for pushing me to clarify here.