Just got through installing a cabin air filter in our 300C. You are correct, it is a bitch of a job, but patience wins out. The housing was already in but no filter holder installed. We bought the whole assembly from Wyckoff for 14.00 + s/h. Sure beats paying ~$35.00 for the filter alone. Don't understand that.
Taking everything apart as you show was difficult because of the @!#$*! plastic retainers. Those are very hard to get out without breaking them or scrtaching something. We broke the two holding the PCM (ECM?) cover on. Removed the cross strut completely and really had to wrestle the old housing out. I had my wife help because of her small hands. I don't see any way to install the filter holder without removing the complete housing and reinstalling it. We reinstalled the new one which fit perfectly. Had to loosen the wiper motor from it's bracket and move the PCM unit out of the way.
All in all, it ended up good and everything fit, but it took about four hours of being patient and holding my temper. My wife is the patient one, not me. Putting the new one in went pretty easy and remembering where everything went back was a small challenge. Finally, no parts left over - yea!
Thanks for your picture instructions. They helped a lot. If I had it to do over again, I don't know if it would take less time, but I don't plan to find out!
Bob