You are correct. The only corrected line affects the proceeds total and should affect the total gain/loss column which looks unchanged. Since the brokerage is not reporting the basis, you will have to use your records to fil that it. That will change the total gain/loss amount anyway. If you don't know the basis, it should be zero.
You can only do what you know is right and keep documentation in case of an audit.
If you don't know your basis, I want to urge you to create a financial notebook that is kept separate from your tax return. Keep it safe and each year, add your year-end statements from all your financial accounts plus a copy of your W2’s, your carryover information, and proof of your basis in your various investments. You must keep tax records from the time you purchase until sold/ loss used plus 3 years. It is very easy to lose track of disallowed losses, carryforwards, and basis.
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