Crickey, its not very often I find myself agreeing with the "Compensation Police" on this board...but I have to tell you, this one is testing me

Follow up your complaint - the delay is probably an oversight/lost letter or an operational delay of some kind. You will likely receive an apology and a few Avios thrown at you and that's about it.

As others have said, its very hard to sympathise with you at all, beyond the fact your original seat was broken. Had you done the sensible thing and just completely moved to a new functioning seat then you might still have had a valid grumble to write in about for losing your preferred seat, but there would be none of these issues with your back. I agree with the other posters that you really don't deserve compensation for that issue specifically, since that would appear to have been caused by some very poor choices on your part.