If the treasurer seems to be "cooperative" and might be willing I would go for the corrected W-2. That probably means an extension, but you should be able to dial in any tax owed.

If the treasurer does not want to correct the W-2, I guess I would probably use the Keogh/Sep/Simple contribution worksheet, and enter the amount somewhere on lines 1-5 wherever it gets you the correct answer.  I would also put an explanation on a "Preparer Notes" page  (f6 PN) it's doubtful than anyone at the IRS will read it, but at least you have disclosed how you dealt with the problem. (I heard somewhere years ago that Preparer notes do get transmitted with the return, and supporting statements do not, but I have no way of verifying that.)

I'm not sure if there might end up being a "matching problem" with a 5498 form at some point so this might not be the end of your issue.