is there a difference between athair and daidí?
Yes, it's same difference as between the English 'father' and 'dad'.
the difference is that your daddy pays for stuff and gives you nice gifts, your father gives some useless advice and the sends you out the door...
What is the difference between daid and daidí, and why is dhaid allowed in this case, but not dhaidí?
dhaid should not be allowed, as a meaning 'her' doesn't lenite. It should either be a daid or a daidí
Got it a second time! Was this word in the notes somewhere? Cause I have no idea what they are doing with the spelling of this word.
The spelling of daidí conforms to the caol le caol agus leathan le leathan rule; no consonant has both a slender vowel and a broad vowel as its nearest vowels.
It wasn't in the lesson. But they slip in new words into the repetition once in a while. Sneaky.