1)
Why did he say "Hashem Hu'?
1.
Rashi: He is the Master, and everything is His.
2)
Why did Eli say "ha'Tov b'Einav Ya'aseh"?
1.
Malbim: He justified Hashem's decree, even though it was primarily because he did not heed the first prophecy. Further, he said that there is a difference between a decree with a Shevu'ah and a decree without a Shevu'ah. The latter is not good in His eyes. He does not desire that it be fulfilled, rather, that it be nullified via Teshuvah. The former is good in His eyes. There is no escape.
