What questions does Social Security ask my employer? They have sent them a letter. I assume it is to determine backpay and confirm inability to work?

Total disability is determined by medical records. How much you will get is determined by past wages over your lifetime. Information asked from an employer is just for dates worked, and wages, if they don't have that already. SS is tied to the IRS reporting of all wages over your lifetime, but they only have each year and wages for each year, not details about months and days.