Feeling like a real dodo bird this morning but isnt there (shouldnt there be) a Standard Report that shows All Invoices and if a Payment has been applied you see the child payment(s) / credit memo(s)? Kinda like the Payments History by Invoice except include Invoices that have no payments... I just sat down with our A/R Manager and she is running 3 reports twice a day and constantly refreshing to stay up to date (most of what we do is Due Upon Receipt). Open A/R, Payments History by Invoice, A/R Aging.

why not just build a transaction saved search for invoice with applied to and applying transaction. Also use formulas, conditional highlight and filter to make it more robust. Over the years I have built many saved search like that, note that reports are less flexible.- You can also try the SuiteAnalytics too.