-- the system transfers only an abstract notion of ownership. It doesn't have the ability to transfer full ownership, by which I mean all of the capabilities, rights, and responsibilities commonly inferred by that term.
Exactly, you got it. The new owner will never know the private key of the previous owner, but the previous owner proved by signing the massege + transferring Bitcoin to the new owner that they are willing to transfer the 'ownership' to the new owner.
It is something new, but it works

I am glad that I inspired you with this idea. Working on it will not be easy and it will be difficult to understand at first, but it seems to me a really promising idea and your project can succeed.
Your discussion of the topic there shed a lot of light on the idea and I think it is starting to become more and more clear, you need to make one NFT at the beginning to try the results and see the advantages and disadvantages.
good luck
Yes, thanks, that you mentioned that someone could try to make an NFT out of it. We don't have a classical NFT here, but this 'signature chain' works and could become something
who knows? Mabe we have something like 'cryptopunks' in the making. At the beginning people didn't understand blockchain & NFT too.