When Maurice runs away, it's as if Clive showing up has poisoned his "fragile" understanding with Alec.

Oh yes, I agree. Clive does come blundering in and spoiling things for them both when they were complementing each other so beautifully. I think that must have kick-started all the worried feelings and the panic about what was happening.

Yes, you're right. I think in the next chapters when Maurice completely misunderstands Alec's first letter is one of the saddest parts of the novel, because Alec is laying himself bare too, but Maurice doesn't see it...but that's for a future discussion!