Yes, you can delete an invoice.
An invoice must first be voided before it can be deleted. See Void Invoice for more details.
Note: If an invoice is marked as deleted, you will not be able to undo the action or retrieve the deleted invoice.
No, all deletions are final.
Deleting an invoice will not have further effects on payments or credit notes. Any effects on payments or credit notes associated with an invoice would have been applied when the invoice was voided.
Deleting an invoice will not have further effects on your financial reports, as voiding the invoice beforehand would have already applied any relevant effects.
Currently, you may not add additional internal notes while deleting an invoice.