Those spark plugs look pretty normal. As for the rust, well that could be down to moisture building up around the lower of the ignition wire. Have you tried dielectric grease?
Do you have the ability to post a video of this ‘slapping’ noise? Piston slap sounds like a hollow sound that is most noticeable on acceleration.
Have you tired to isolate the engine noise by disabling 1 cylinder at a time to see if your engine noise disappears? Thatis one way of checking for a rod knock and so forth.