Apparently at some point I viewed the と and が N5 grammar lesson and it's marked with the star that means learned. Can I unlearn it so everything works normally when I actually officially reach that lesson?
There is not a way to do that, no - however, you can always return to that lesson from Resources > Japanese Lessons in the menu (as you most likely already know).
There used to be a time when I had halfway gone though a lesson rerun that it caused the star to be a half star or unmarked in some way. I think that's somehow been patched, unfortunately.
But in terms of schedules, you can just make them separate schedules and then freeze that for a later time if you want.
You can always go back to review lessons and schedules whenever.
