1)
How did Shmuel know that Hashem did not choose him?
1.
Radak (9): Hashem said (verse 3) "the one whom I will tell you"; Hashem did not tell him to anoint him.
2)
Why does it say regarding Eli'av "Mi'astihu", and regarding the other sons, "ba'Zeh Lo Vachar Hashem"?
1.
Pesachim 66b Hashem initially desired Eli'av, but rejected him. This teaches that if one gets angry 1 , even if greatness was apportioned [but not yet given] to him from Shamayim, he will lose it.
1
Rashi 66b: Even though the anointing preceded Eli'av's anger, Hashem knew that he has the disposition to get angry. We do not find any other shortcoming in him!
