So the empty passenger seat rattle is also due to the headrest. If you wiggle it, you'll notice it has quite a lot of play especially if it's not sitting flush against the seat and has been raised up. If you pull it out completely, you'll notice that rattle going away.
Another issue is the plastic tabs used. These do come with the foam rings on the front part where they clip into the metal panels and act as a decoupler but if the back end loosens over time due to the car being driven on rough roads, the said clip will rattle on the plastic trim side rather than rattle on the metal side. You can easily get a pack of 100 clips for 500 and swap them out
For plastic pieces that creak and squeak on movement, you can either add some kind of decoupler foam or tessa/cloth tape and increase the mass by adding some cld sheet pieces