What is the significance of the fact that neither they, nor their fathers had known about the Manna?
Ramban #1: The fact that one can live on the supernatural Manna for a long period of time was a Chidush (a novelty) that neither they nor their ancestors would have conceived possible. 1
Ramban #2 (citing the Midrash Rabah): The Torah is pointing out tremendous Chesed that Hashem had performed for the first time 2 by feeding them Heavenly grain, something that neither they nor their holy forefathers 3 had ever merited before. And this teaches us that, since Hashem can sustain man in whichever way He decrees, we should keep His Mitzvos in order to live.
What are the implications of "Va'yeancha, Va'yar'ivecha Va'yachilcha es ha'Man ... "?
Ramban #1: They all refer to the Manna. Refer to 8:2:2:1.