Read your policy. It details the insurers obligations to you. After you have read it, if you feel they are proposing to breach their contract with you, you can let them know that. If they insist on doing something that you think breaches a specific clause of the contract you can sue for breach of contract. Most policies do not cover the cost of repairs, but the difference in the value between the damaged car and the undamaged car.

You have your estimate. You sent it to them.
Of course they are going to refer you to some other place where you can get a new top for less money. That is the way they operate.
Did they come to assess the damage?
If not, demand that an adjuster come to see it.
Have you the receipt for the stereo?
Make sure they receive that also.
Yes, keep fighting for a replacement top based on the estimate you obtained yourself.
Keep a record of what has been represented to you by the insurance company since the incident occurred.

Ask them for the name of the body shop that will repair the top for $600

Also, whats the deductible for your comprehensive insurance ?

That deductible you have to pay yourself.