Can not answer “Yes” to all three questions; as payments from clients is not enabled for the client control panel as no facility for online payments. The clients are notified of the bank account to transfer their payments to on the invoices and they make the payment through their own bank accounts, or send a cheque.
Would there be a way to inculde an opt-in to Google Reviews when sending the customer confirmation of their payment on the Client Payment Slip?
This isn’t something that we would be able to do (at least, in the same fashion), as the page itself needs to be on the domain name of the merchant website.