Q1. Where can I adjust the settings for a single customer credit note?
Click on the gear icon on the right side of the screen while creating a customer credit note. A menu should show up.

Q2. How can I remove VAT, item discount, or item quantity & unit from appearing in the customer credit note?
To remove VAT, you can adjust the default GST/VAT settings to No VAT.
To remove item quantity & unit, you can disable both options under Item Attributes.