1)
Who are Pinos ha'Am?
1.
Rashi: They are the officers. Even Pinah (the cornerstone) is the [most] important.
2.
Targum Yonasan: They are the leaders. Radak - they are called Pinos, for they are to the nation 1 like the corner is to the wall.
1
This is the correct text of Radak. I.e. they support and strengthen the nation. (PF)
2)
Why was Sha'ul sure that the sin was today?
1.
Malbim: Had the sin been earlier, Hashem would not have done such a miracle and salvation today. Surely the sin was after the miracle.