You are doing the right thing by complaining to the ombudsman. They haven't justified the offers made to you with any evidence so far as I can tell.
Regarding buying the car back - they are under NO obligation to sell it back to you. Once they've settled with you they own the car, it's up to them to decide what to do with it.
I wrote my car off last year (link below my avatar) thankfully they offered a very fair amount (only £245 less than I paid for the car 18 months and 25k miles ago!). However, to buy it back (which I did) I had to bid on it at an auction and it cost me £800ish (33% of the amount they paid me)
I had a good chat with the guy that valued my car, he said it was one of the most difficult cars he'd ever had to value. At the time there were only 4 or 5 Of the same cars for sale, all on higher mileage, automatic and with substantially less history. They offered me more than the prices of the cars for sale at that time. Clearly some insurance companies are better than others!