I had this problem. Stef get the car running and then get someone to go lock to lock on the steering while you look in the pas reservoir with a torch. if you see bubbles you have air in the system. I had this a week before going to Donny last year but it cleared itself

I just kept on giving it lock to lock treatment and it gradually got better. Yours sounds as though it has been around for some time though
Have you had the car supported such that the front suspension drops to maximum extension for any time? If he seals to the rack are not that clever imo this could allow air into the system.
Next thing is to disconnect the discharge from the pas pump and see if you have a good flow. Warning this can be a messy business so be careful.
You're right though, if the steering will not centre and it will be to stiff to drive you would end up with arms like popeye. You can get racks refurbished but the aggro here is getting the thing out. I do hope it all works out for you so you don't miss the ring
