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