1)
What is the meaning of "Nafsho b'Tov Yalin"?
1.
Rashi: In the grave, it will be good for his Nefesh [Radak - if he stays on the path that Hashem will show him. This is the primary reward of man!]
2.
Malbim: His primary efforts are for the happiness of his Divine Nefesh, which will dwell in good in the world to come.
2)
Why does it say "v'Zar'o Yirash Aretz"?
1.
Radak: Due to his virtue and straightness, Hashem will bless his seed after him due to him.
2.
Malbim: His only desire from this world is to perpetuate the species, so his seed will inherit the land, i.e. succeed in his merit. He himself is satisfied with the most meager rations.
