The "air spring" was more than compressed enough, there simply wasn't enough clearance to pull the thing out. The gap between the body and the rear arm that the spring sat on was less than the diameter of the metal part of the bag so there was no way to get the new bag into place even if I butchered the old one up to remove.

Using a long piece of Hardwood to lever between the body and the suspension and push the hub away from the body was the only way I could make enough room.

I solved the clip problem by taking the factory clip and sanding it down so the clip was flat at the bottom. After that the clip went home and problem solved!

I pulled out a Bilstein unit from the left rear with a build date of 2019 which looked excellent. The car would be lucky to have done 10,000km. Would it be worth selling?