You may use the class as a departmental account. Set up marketing, R&D, production, etc. as a class; then, tag it to each income or expense account when creating an invoice or bill.

Renamed the accounts receivable to trade debtors (or trade receivable) if you prefer to separate trade from non-trade. Then, add other debtors or inter-company receivable to the account list for tracking non-trade debtors or subsidiaries.

if both invoice and credit note is in a different currency, then have to consider using a contra account to clear them instead.

You can manually change the document number on the go when creating an invoice if the software administrator has set the sequential number preference as changeable.

The data has to be in rows and columns (like a CVS format), usually exported from a quotation or a CRM system, to import into an accounting system.

Lock your valuables in safe.
Lock your sensitive document in the drawer.
Lock your house away from the unwanted visitor.
Lock your accounts to prevent data fall into the wrong period.