You are looking at this from a position of income vs the IRS which sees people writing off business losses that really should not be. The 9 factors help to determine when the IRS can say the losses are not deductible. If a person is regularly engaging in an activity and making money successfully, putting in time and effort, knowing what to do to keep making money, profit in the past and expected profit in the future are a good foundation for a business.
If the teacher does it one time for kicks, it isn't a business. Knowing how to teach and expecting to continue, makes it a business and allows the deduction of the books.
I agree with @GeorgeM, this is a gray area and you have done your research and analysis. If the IRS decides to audit you, you have answers ready. You are reporting income and paying tax rather than writing off tons of expenses and claiming a loss.
@rickwm
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