Why does it not lenite?
It should mutate to dy liniadur. Flag it as an error when you see it again.
Yes, it's a mistake, to be corrected.
It should mutate after dy
Why is 'ydy' incorrect here? As in: 'Ble ydy dy liniadur di?'
The question words ble?, pam?, pryd? and sut (when it means 'how?') should always be followed by mae rather than yw/ydy.