Currently Books supports multiple contacts to be assigned to the portal access. Estimates, Invoices etc can be made available by manually selecting the right subset of contacts. However, by default, the name (which is full name, not just first name - please fix this) is always of the main contact in the email body.
Second, the automated reminders should go directly to the person associated in the portal with the invoice, not the main contact. In particular dealing with larger organisations, it is imperative the invoice lands at the desk of the right person, or payment is delayed.