Romans 9-11

Who are counted as the descendants who are the children of God?

Not all who have descended from Israel are Israel. (Romans 9:6ff)
Clearly Paul writes that it is not the naturally born children who are God’s Children
(the Jews) but the children of promise. The children of promise are those who have faith in God through Jesus Christ. Definitely it is not by works but by God’s choice. And God has chosen that those who trust Jesus are God’s children. Paul also writes in 10:12 that there is no difference between Jew & Gentile. How is it then that some talk about a different plan of salvation for the Jew over the gentile. Specifically that the Jews will have another chance to accept the Messiah (Jesus). How can this be? The message of the gospel (10:13) is that all who call upon the name of the Lord will be saved. Chapter 10 definitely portrays the Jews as being obstinate and refusing to believe in Jesus.

From where does faith come?

Faith comes by hearing the message through the word of Christ.

Who has received God's mercy because of Israel's disobedience?

The Gentiles. Paul reckons that the gentiles salvation ought to motivate the Jews to accept what the Gentiles have accepted.