The AR invoice entry screen knows nothing about the inventory module.  Those instructions may not apply to your situation. 

For corrections I'd typically recommend a credit and new invoice.

Do you typically create a credit memo to zero out the previous invoice & create a new invoice or just create a credit memo for the difference?

For example, I have 10 customers that we need to correct. Some of them have 5 line items on the original invoice and some of them have 50. Only 1 of the line items in each of those previous invoices is wrong.

It depends on if you need a fully correct invoice to send the customer, or if the CM does what you need.

It depends on if you need a fully correct invoice to send the customer, or if the CM does what you need.