Leatherique will help with the lighter areas, it will take multiple applications (8-10) to get the leather all black again. Follow the directions on the bottle regarding keeping the interior warm while the product is working. I found it was much easier to apply the rejuvenator and the cleaner by swapping them into spray bottles. The downside is you really need to allow this stuff to work over the course of a day or two reapplying as it gets soaked in; you won't be able to drive the car while it works it's magic. I used it on some very dry and badly cracked seats in my e34. The dryness and the light areas of the cracks are totally gone. The leather is still cracked but it's very soft and all black again. IIRC it took about 12 applications to reverse 20 years of dryness.
I think thats a bit too far gone to ever hope to really mend. I would talk to an upholstery shop, maybe pick up a set of extra seats and have them install some new leather on them and if they do a good job give them the door panel too.
Definitely fixable, depending on which approach you wish to take. I would try the leatherique fix, and if the doesnt work check out furniture clinic. You can recolor the seats and bring them right back to life if you want a fun DIY project.
I would definitely try Leatherique first. It will take at least 10 cycles, but you will be amazed of how well it works. The first two are the passenger seat now, the other is the seat from the previous owner's photo stream. Now the seat was dry and heavily cracked before treatment. Now it is very supple, but you can see the cracks are still there but all of the white areas are gone. The flash makes it look like there are some lightness in the cracks, but they are black as the rest of the seat under normal light. Since its doesn't appear your seats are as deeply cracked as mine, Leatherique may work well enough to delay new leather for a while.
I've made 1 cleaning attempt so far -- 1 pass on the entire interior. The more I looked around the more I noticed little marks here, there, everywhere. I wasn't expecting too much since it all appeared to be wear and not stains, but some of the white areas are a dull gray now. So there's a slight visual improvement.
Can I use this stuff on the headliner? I haven't noticed any spots, but I'm asking just in case.
I'll probably attempt 1-2 more times to clean the spots/wear before moving on to the conditioner.