1. You can always fix anything in your headless branch. It seems obvious, but... you don't just have to follow their rules and step by step guide as they move through the code. If you make a mistake as you're going through the rebase / commit conflict changes, you can always go back to the file and fix the changes that you made earlier.
2. If you are in the middle of a rebase, and you need to leave in the middle to go do something else, you can do git stash, then rebase abort. Later, when you need to come back to it, you can do git stash pop. Git stash is its own world, and you can save any changes there that you want, regardless of which branch that you are on. (Even a rebase headless branch).