No Christians that I know of dismiss the Old Testament.
The problem for Christian Zionists/dispensationalists, however, is that they have myopia when it comes to Bible study — they cherry pick verses in the Bible and apparently dismiss others (forgetting that Scripture never contradicts Scripture). I believe you just gave a good example of that — interpreting Romans 11:25 in a way that makes it contradict Romans 9.
I think if you would pray for understanding and read Scripture in light of Scripture, then you would realize that the modern state of Israel is not a fulfillment of Bible prophecy.
YHvH's larger plan of salvation, where His People are blinded for a time,shalom b'shem Yah'shua
in order to bring His salvation to the gentile nations.NAsbU Romans 11:15 For if their ( His Chosen People ) rejection is the reconciliation of the world,And their ultimate salvation after a period of blindness is guaranteed by YHvH's covenant :
what will their acceptance be but life from the dead?
NAsbU Romans 11:25 For I do not want you, brethren,
to be uninformed of this mystery--
so that you will not be wise in your own estimation--
that a partial hardening has happened to Israel until
the fullness of the Gentiles has come in;
26 and so all Israel will be saved; just as it is written (in Isaiah 59),
"THE DELIVERER WILL COME FROM ZION,
HE WILL REMOVE UNGODLINESS FROM JACOB."
27 "THIS IS MY COVENANT WITH THEM, WHEN I TAKE AWAY THEIR SINS."
28 From the standpoint of the gospel they are enemies for your sake,
but from the standpoint of God's choice they are beloved for the sake of the fathers;
29 for the gifts and the calling of God are irrevocable.