Isn't it possible to implement ecash in a way to break this? How do you know if a custodial wallet implements it like that or not?

Thats a powerful feature that sometimes people refuse to see

Develop this to me as if I am Peter McCormack pls

Since I watched your interview with Peter McCormack you are always in my radar. I also studied Physics with MS but stopped before going to PhD route so I really admire you. But even after watching the show it is not clear for me how to use Ecash, cashu etc... I got the same problem with Obi for exam...