An ABN can be signed, but it must be done before services are rendered. Can't do one after they are rendered.
As far as billing the patient goes, what does the EOB say for patient responsibility? If it says the charge is patient responsibility, you can bill them.
In regards to changing what was billed as the primary dx, you have to go with what the documentation says. If the reason the patient presented was keratitis, with a secondary dx/finding of Optic Nerve Hypoplasia, then that is how it is. You can't switch dx order because one will be paid, and the other won't.