GL impact - totals approx $9100

When you enter a standalone invoice (not created from sales order) the inventory impact occurs on the invoice rather than the item fulfillment. So, in addition to having an unexpected GL Impact, you will also find that the items you used on the invoice have had their inventory level/value impacted equal to the quantity on the invoice. The same is true of standalone bills, bill credits, cash sales, etc.

Hey Nick - that is exactly what happened - NetSuite PS imported historical transactions or attempted to and left the GL impact after all while importing historical trial balances as well. So I have largely corrected Sales. Now unraveling COGS.

That's a pretty big fail on their part tbh. They should have been using some non-inv item so it wouldn't affect inventory/COGS. Your inventory balances are going to be messed up as a result of this too. I'd consider pushing it back to them to fix.