Is it possible to reuse an add-on’s ID and preferences among a demo and paid edition of the app? (Assuming the paid version were distributed outside AMO but as signed copy.)
Yeah I think you can, but you might want to use a different one so it doesn’t pollute your statistics dashboard by mixing in the paid results. Unless you want that. But im not sure if you can upload to addons with the same id, try it out, it will error in the upload process and tell you.
I’m not sure I understand the question. If you want to distribute outside of AMO, soon you will need it to be an unlisted but signed by Mozilla addon. They will not allow the same ID for two addons.
Different add-ons can share preferences, yes. Different add-ons will need different IDs, but there’s nothing preventing them from sharing preferences.