2. Multiple estimates are required after an accident. By law, you are only required to get one estimate. If you want to get multiple appraisals, that's fine too.
3. The insurance company and repair shop may not agree on a price. If they don't and the insurance estimate is lower than the cost of repairs, you must pay the difference. Your insurance agency and repair shop should agree on a fair price. If you choose well, your repair shop will advocate for you and provide evidence of all the damage sustained. Collision insurance coverage should cover the entire cost of the damage. You should not pay anything but the deductible.
4. You must use the repair shop with the lowest bid. As I've noted already, you can choose whichever repair shop you like and you don't have to get more than one bid. Your insurance agency may push you to go with the lowest estimate, but you should choose the repair shop based on their knowledge, expertise, and customer service, not their low price. Low bidders rarely have the best service.
5. After your insurance company writes you a check, any additional costs must be paid by you. The insurance company should pay for everything (except for the deductible). The estimate was only an estimate and if additional repairs are needed, your insurance company should send you another check.