How and when should I include a historians' opinion?

You should include a historians' opinion to support or contrast the argument you have made. For example, say you were arguing for a certain time period that agrarian issues led to public discontent, and thus provided an incentive to rebel. You would support this with evidence, and either find a historian who supports this, and explain how this answers the question, or use a contrasting opinion that supports the judgement you are going to make. So if you believe that it was not actually agrarian issues that incited rebellion, but rather unfair taxes, then you would explore the argument for the agrarian issues, but conclude in your paragraph, with a historians' support, that it was far more likely to be due to unfair taxes, and give evidence to support this.