Woah. That's unfortunate. And the reply from the QB rep is even more unfortunate.

If you were my client, what I would probably do is take the bank statement from 12/31/2017 and try to reconcile the bank account to that bank statement. Check the box that said "Hide all transactions after the statement date". Then select all the charges from the beginning of the account through 11/31/17. Then go through an individually select all the charges for the Dec 2017 statement. With any luck, you can reconcile it through that date. If worst comes to worst, enter your own transaction (check/deposit) to get the account to reconcile to the 12/31/17 statement. Its not ideal, but at the end of the day you can't spend months trying to re-reconcile the whole account and your taxes have already been filed for 2017. After you reconcile to the 12/17 statement, go through all 12 months of 2018 and reconcile those by month. I would say its more critical to get those reconciliations by month in QB since you may still need to file your 2018 taxes.

If you have already filed your 2018 taxes, you can try to method I mentioned above but use the 12/31/18 bank statement. Get the 2018 balance sheet from your accountant and enter a journal so that your balance sheet and P&L match their records.