When a Form 1099-MISC is received, the IRS expects to have it reported on Schedule C for self-employment. By reporting the income on Schedule C, you can also include any expenses that you had related to that income. This will reduce the income from the Schedule C that would be subject to self-employment tax. Without the Schedule C, the IRS assumes that all of it should be taxable and subject to self-employment tax.
You say that it was a one-time compensation, but for that one year, you were operating a business and providing a service for the money that you made. The only way to show that it is not taxable will be to show that you had expenses related to the income reported on Schedule C. This will reduce or eliminate the self-employment tax on the income reported on the 1099-MISC.
You need to follow the advice in the letters you have received from the IRS with regard to what they want from you. The best way to go about providing the Schedule C for the IRS would be to create an amended return and add the Schedule C. Then, you will have a benchmark to know how your original return has been changed by changing the way the income was reported.
The IRS may or may not want the actual amended return, only the Schedule C may suffice for their records.
Here is a TurboTax article to help you with amending your 2017 tax return: How do I amend a 2017 return using TurboTax?
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