I don’t know anything about Scheme and have not written an addon yet but I also update all my Ember CLI projects by git merge form a clean up-to-date Ember CLI (ember init .. overwrite all) branch. I use the same remote for all my projects.
I think it is a good way but I also think it will be hard for a beginner to understand git merge , git fetch and branches. Perhaps we need to do something like “hub cherry-pick”.