Can’t really tell what the two pictures are trying to show, and going back to your original question, I don’t think back flushing will help because it’s what’s in the sea water that is the problem.
I’m sceptical about the benefit of replacing the inlet pipe, but I do know that once it is cleaned the odour problem goes away. What we need is for someone who changed some time ago to ‘odour free’ pipe to comment.
