I'm such a numpty!!
I put this to one side for several days due to other
other more pressing tasks. Getting back to it yesterday, I solved the few remaining clues (although I can't parse 5d—does it involve an intermediate definition?) and ended up with 7 compass points...
After wading through 100+ post here, I discovered that "snakes" doesn't necessarily lead to a word ending in a superfluous "s". I worked out the compass point pairings weren't diametrically opposite ones, drew the lines then got the scissors.
That's where it all went pear-shaped!
I convinced myself that the preamble required me to use the four smallest piece to make a smaller version of the largest piece — even though I knew that I needed two of the smaller pieces to provide the missing letter from each of the actors's names.
Sitting here just now, idly putting together the pieces that created two actors' names and joining them together to create the film title, I finally had my PDM!