I feel like I’m taking crazy pills 5 days before the exam, but somebody more sane please help.

Just by the way a linear trend model is formed, y = a + bt, isn’t it assuming no mean reversion? So if you have a mean reverting time series, your linear trend model will not be helpful and presumably have low explanatory power. Now if you have a time series that is NOT mean reverting, the linear trend model seems good but then an AR model doesn’t work since it isn’t covariance stationary (until you first difference).

Is this the correct way to think about it? Mean reverting time series = don’t use linear trend model?