So, even with the Sigma sig, one can add items to any colle…

That's right. The indexer only verifies that the items into a collection have the same collection ID, which matches with the Ordinal ID of the cover, and individually verifies that the Sigma signatures are valid on each item. However, it does not verify that these signatures on the items match the Sigma signature of the cover's issuer. That's the weak point of the metadata structure in 1Sat Ordinals protocol.
The limitation of only being able to add new items to a collection if you own the collection cover is a feature of the NFT platform, but not a limitation at the protocol level.
For example, we created this collection where we shared the cover (by transferring it) with several artists, and each one added an item with their own Sigma signature, while the cover image's signature belongs to the original creator. Let me know if this is clear.
https://3dordi.io/collections?search=a16e7e05c2ecd031d992642d047e92524d55cfec7f21006644a9aa30b78a8c56_0

An expert agent could inscribe new items within a collection without owning the cover image, simply by knowing the collection ID and including it in the transaction building. However, it's possible to verify if an item is counterfeit by checking the inscription date or comparing the creator's sigma signature or the artist's digital identity address. That's why I've chosen to use the Ordinals address (for default) as the digital identity address in 3DOrdi for better identification of NFT creators by consumers.
Therefore, we should implement a double sigma signature structure in the future: one from the creator artist and another from the platform, to provide greater reliability. Similarly, indexers could improve their traceability logic by comparing the item signatures with the cover image signature, but they don't currently do this, either because they haven't identified it as a risk or because the process introduces complexity that can be handled by Ordinals apps and platforms.
My ultimate goal in opening this discussion is for creator artists and collectors to give more value and find more utility in the individual sigma signature of artists linked to their digital identity.
For instance @ZoideNFT, doesn't use the individual signature scheme of the creators but always uses its platform signature. In contrast, [[3dordi]] hasn't introduced its unique platform signature but instead gives this added value of individuality to the collections creators. The future is to introduce both signatures in each collection for all marketplaces.
https://docs.1satordinals.com/adding-metadata/signing
@KURO @pxl272 @SMARTWATCH @RosaAmargada @Bsvcrypto @Ralf @4DToken @ChefMyKLove @metamitya

Great info.