I can help you with resolving the error when trying to connect your bank account, @unassailable.

Can you try to check how your bank downloads your transactions in QuickBooks Online (QBO)? 

Please take note that you can’t connect both a parent account and its subaccounts. If you try to connect both, you’ll get a message that the account is already connected.

I'm also adding this link to learn more about adding multiple accounts: About bank or credit card subaccount setup. It includes connecting them to online banking.

See this how to reconcile accounts article for a step-by-step guide in reconciling your accounts. 

Let me know if you have additional concerns about bank account. I'm always right here to help you!