Greetings from Canada ignaceiiI agree with all your points that are factored into a piece being "learned". However, I would add one more factor to your list: you are able to perform it in front of an audience.Performing a piece in front of an audience is a test of what you've been working on. You only have one shot when performing; there's no going back once your fingers press the keys. If you are able to execute the piece in front of the audience at the same level (more or less) as when you're practising it, then I would consider the piece to be learned.
One other thing I might add is that you may never consider a piece completely learned. I like to think that pieces I learn will be played off and on for the rest of my life and be improved each time.
On your scale, 4 for memorisers and 3 for readers is the place where you start to learn a piece, not where you are finished.
What do you mean by memory work ?Analyzing structure harmonically ?Note by note ?It would be interesting to understand the process of memory work. With or without the piano.
I think he means that learning a piece, even if you're using the sheet music, is all about memory. If it weren't (eg if you had no memory), you'd just be sightreading it as if it were the first time you played it, every single time.