RADAK
1.
This is the generation of them that seek Him...
Who shall ascend into the hill of the Lord and in His Holy Place, because he is clean of hand and pure of heart?
This is the generation of the seekers of the Lord, and they are Israel.
And this happened in the days of Solomon, who built the House. And the KETIV דרשו (him that seeks Him) corresponds to דּוֹר (generation), which is a collective term, singular; and the KERI דֹּרְשָׁיו (them that seek Him) corresponds to the individuals, who are many.
[NOTE:
keri (the way a word is read)
ketiv (the way it is written in the actual scroll)
]
2.
...That seek Your face, (even) Yaakov
The singer speaks as to God. These are they who seek Your face in the Holy House, Yaakov, and they are Israel; for all the seed of Yaakov are called by the name Yaakov, as (Yirmiyahu - Jeremiah 30:10) "And you, fear not, My servant Yaakov" with other instances besides; And there is no need to supply - "seed of" as the interpreters have done.