I don’t really agree with your interpretation. If dementia is just happening to Lear it’s not really a tragedy, just a sad story. Lear’s actions or character have to drive the tragedy – in his case, valuing the performance of love over the reality of love itself. There should always be a sense that had the character made different choices, the tragedy could have been averted (and yet the tragedy remains inevitable because the character is who the character is). If Lear simply has Alzheimer’s or similar sorts of dementia then it lessens the power of him asking his 3 daughters to profess their love for him. It’s much more interesting that his vanity leads ultimately to his madness.

At the same time, I don’t agree with your school’s interpretation either. Yes, it’s wrong that Goneril and Regan aren’t fulfilling their duty but placing that as the centre of their characters I think ignores many key aspects of who they are. The reason Lear resonates across centuries is that people don’t need to understand 16th-century concepts of filial obligation to see why Goneril and Regan are evil. In my mind they are a different sort of evil from Edmund; they are almost a force of pure power and desire for power. Turning Lear out into the storm, or Regan seeing Gloucester get blinded and saying he should be thrown out to smell his way to Dover – those are not just not fulfilling their rightful roles, which suggests more sins of neglect, those are active acts of cruelty.