I have removed the entire interior of my 2001 Eldo, seats, console, carpets,door panels, trim pieces, dash,and rear panels, along with the rear shelf and light. Everything except the headliner. As I did it I ordered all new clips as the ones I removed were usually damaged. Also, you have to be vigilant and quite knowledgeable as there is so much nylon/plastic, that you can destroy without a forewarning.
Once you have everything out, CLEAN, do the leather treatment, and photograph EVERYTHING in EVERY WAY!!!
I Dynamated the floor pan and part of the side walls - man, this car has a quiet ride now.
You will find the rear panels are solid, and made with vinyl over foam and plastic or perhaps fibeglass. Fix what needs fixing aand when you replace everything, take your time, do it in the correct order (RTFM), and again, use new clips. And photograph every step from every angle. Digital photos are cheap, and you will will reveiw them often if you keep the car.
Plastics (remember the Graduate?) shrink with age, harden when exposed to thinks like body oil, evaporate and crumble. Leather will last forever unless mistreated, exposed to heat, aand UV light. So use treatments regularly to help prolong the enevitable.
I noticed some guys at cars shows were using "near leather" of something like that which is cheaper that leather and is man-made, for their door panels and trim. It's claim to fame is that is lasts a long time, is not affected by the elments like vinyl or naugahide, and is easy to maintain. Just need to find someone that can match the interior panels in the Eldos and Sevilles with this stuff.
Happy Trails. Good luck with your interior.