I just asked o3 how to fill out a form 8949 for a sale with an incorrect 1099-B basis not reported to the IRS. It said (with no caveats or hedging, and explicit acknowledgement that it understood the basis was not reported) that you should put the incorrect basis in column (e) with adjustments in (f) and (g), while the IRS instructions are clear (as much as IRS instructions can be...) that in this scenario you should put the correct basis directly in column (e).