1)
Where do we find that Hashem promised that David's seed will endure forever?
1.
Radak: "U'Vayis Yivneh Lecha Hashem" (Divrei ha'Yamim I, 17:10) - this 'Bayis' is sons and grandsons forever.
2)
Why does it say both "Ad Olam" and "l'Dor va'Dor"?
1.
Malbim: "Ad Olam" is a connected time period; "l'Dor va'Dor" is time divided into generations. David's seed will endure forever, without interruption. However, his throne will be Batel at a time of Galus. Hashem will return it in a generation that pleases Him.
