I know what you mean about overspicing, some of my gran's onions were not for the faint-hearted. ;D
You could try pouring some of the vinegar off, and replacing it with a sweeter vinegar, such as sherry, and perhaps including a bit of medium sherry. Might be a bit expensive, but would be worth a try, especially if you consider them inedible now, and it would possibly save your 'crop'. Best I can suggest. Good luck,
valmarg