Insurers do not tend to dispose of salvages until settlement has been agreed. I would contact the insurance company and ask for the car seat and stereo back. Insurers move cars from recovery yards as they charge storage and move them to their own salvage yards. They should be able to contact the salvage yard to see if the items are still in the vehicle and make arrangements with you for them to be returned.
If the items have gone astray I would write a letter to the insurer making a complaint that they were stolen whilst the vehicle was in their possession.
Sorry but not heard of an insurer advising that you can't submit any evidence regarding valuation. Usually if there is a disagreement you look at Auto Trader. For the insurers offer you should be able buy the same make, model, milage, age, condition etc car from the private cars for sale section to check the valuation is there or therebouts. (Sorry not garage prices as they usually include a warrenty. Insurers consider this betterment as your own vehicle didn't have one.) If this shows the offer is low you submit the adverts to insurer asking them to reconsider the valutation.
If they refuse to reconsider the valuation and your adverts show the vehicle was worth more again I would make a complaint.