The purpose of a barcode is to make products identifiable by scanning devices. If there is no one-to-one relationship between the barcode and the product to be identified, the barcode is useless for this purpose and you need other or additional methods to identify the product. This is why there is only one barcode allowed for a product. If you are using the Odoo POS, you can identify products manually using the POS user interface.

ad 1): If a product has multiple barcodes, what is the meaning of these barcodes? Use the one that is identifying the product, if they are distinguishable, if they are not, they are useless.

ad 2): I suppose you mean that the product has the same barcode as the packaging. You decide which one is the stockkeeping / selling unit.

ad 3): What for do you need a barcode on a packaging? See 2).

ad 4): If you must identify product variants, then, as already said, the barcode is useless in this case if it is not relating to the variants.

ad 5): If a barcode is not readable, identify the product manually.

ad 6): Don't use the barcode for identifying a product when it is not able to identify a product, as simple as that.

There is no need to use barcodes. Odoo offers multiple ways to easily identify products manually. If you want to use barcodes, make sure they fulfill the requirements, means are able to properly identify products.