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It appears that you may not qualify for an interview waiver because you had an H one B denial that was never overcome.
There is no specific format for a non-availability certificate from the local administration. It can be a letter with the official stamp, a letter on their letterhead, or a specific form that the local administrations have created for their own use. You should get one.
I do not see how an aged-out child can travel on a derivative visa.
The employer can start the green card process at any time, even before you join. To see the salary figures by county and profession, you can review this link - https://www.flcdatacenter.com/