I just got off a lengthy chat session with the QB team regarding this issue.

There are two ways to import invoices after setting up sales tax:
1 - Delete the account and open a new one. Don't set up sales tax this time
2 - Pay an additional fee to access a third party app that fills this gap in the QB functionality. Even though a bug created the need for this app, QB will not offset the customer cost associated with this solution.

We don't have much data in yet, so we'll probably choose to delete the account and deal with the refund process.