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Here's a link to the MacOS version layout for alt-codes, which I believe handles all your points except the last: https://i.stack.imgur.com/v1ZLm.png

Thank you for the clarification. If I understand correctly, one needs to press two keys to print « É ». Do you agree?

If it's right, I believe that it only invalidates point #4.

Shift + /, when using fr-CA, does return É. Is that what you meant with "two keys". For œ or Œ, I use Option+Q or Option+Shift+Q (Mac Big Sur).