R.F.
As a parent of two twenty-somethings and one still teenager who between the three of them have totalled more cars than I care to count, I can tell you that the adjuster will be telling you what will happen -- not the other way around. It sucks, I know!
Yes, the repair center will most likely use "used/salvaged" parts depending on the age of your vehicle. Your insurance's only requirement to to repair your vehicle to the value it was prior to the accident. Therefore, if using salvaged parts will get the job done, that's what will be used -- rather than new.
Now, with that being said, if you're using a reputable repair center, you will more than likely be satisfied with their work. That you do have rights about. It's easier to use a shop that will work directly with your insurance carrier and generally better; but not always. However, all reputable repair shops have to guarantee work they did on an insurance claim.
Visit with the adjustor. I have a feeling you will be satisfied with their response. If you're not, call your insurance agent/carrier and speak to the adjustor's supervisor until you are satisfied.
Good luck. I know this is hard. Been there too many times to count!! :))
R.