It would be helpful if one-time after a substitute user is setup, both the substitute user and person they are substituting for receive an email to confirm the setup and period. This may help avoid any confusion around the time period and ensure they are on the same page.
Thanks for sharing this idea. It makes a lot of sense, and is in fact, in-line with some of the enhancements we are thinking about for our notification framework. When we get around to planning this, I would love to reach out to you for getting additional inputs.