Yes, you can edit it in the product page and it will override the automatic one.
Yes, but only if you have enabled the option in the plugin settings dashboard. For further information, we recommend reading this page.
Yes, the plugin supports variable products. You can set a different barcode for each product variation.
Yes, you can do it. Go to the Tools section of the plugin settings dashboard in YITH Plugins -> Barcode and QR Codes, and click on the Apply barcode button.
The plugin does not support this feature natively, but you can do this using WooCommerce built-in export tool or an external plugin that export products, the barcode will appear in the export file as well.