Any reason why the intercompany payables vs receivables accunts would not match up. Besides transactions and currency revaluation reasons?

@Ogabeg Sure, all kinds of reasons. One-sided transactions via Journals, for example. You don't HAVE to make balancing entries.

What would be the best way to zero out both accounts with one entry as of May 2019?

@Ogabeg Well if they don't balance, by definition you can't. You'd have to make 2 local entries. If you want to be top-notch you'd have to hunt down the transaction(s) that broke the balance and edit them to correct them. But that likely involves reopening closed periods, etc.