This is why copyright infringement is settled in the courts. It is rarely black-and-white so there is judgement involved. The case of LVC was pretty easy because the script was about a summer house, but the designer decided to use a miniature house and use squares of light to represent rooms. This is probably why the Caldwell setteed--the infringement was obvious since the design worked against what the script described and would be a slam dunk in court for the plantiff.
But most design infringements are not that obvious---hence the need for a court to make a decision. In IP cases it can be very grey.