Dear Ms. Kets,
Unfortunately If you give a business user access to "CRM_ITEMSTOBEINVOICED" - CRM_CUSTOMERINVOICING , they can create a credit memo or customer invoice.
your understanding of the system is correct but this is not supported in current version of SAP business ByDesign.
you will have to create an add-on/custom solution to full-fill your requirement of "approval of credit memo" or access context for access restrictions for creating customer invoice or credit memo.
Regards
Leo