This is sophistry. The OT Jews were God's chosen people.
True. But not all, since not all believed nor obeyed the Lord God of Israel, but cast aside His law in their works.
Jesus was a Jew, as were all His disciples and all the Prophets,
True. And His 12 chosen apostles, including Paul after Judas fell by transgression.
so saying that Jews don't believe that Jesus was/is the Messiah is nonsense.
Which is what Romans 11 is all about. After the cross the Jews were the first to repent and believer His resurrection, and be grafted in again to His holy green olive tree.
And I am a Jew, and I know (not believe) that Jesus is the Messiah.
Congratulations brother! On being a believer in Jesus. Not in being a Jew, since Jesus could care less. Nor me. Except of course on some cultural level.
Jesus, when He was on earth as a human, was distinct from God, the Father. Otherwise He would be praying to Himself, which is absurd.
True. The Son was the true God on earth in the flesh, while the Father was the only true God in heaven on the throne.
You need to read Romans 11 to understand the truth about God's chosen people.
We can read any of the NT after the death, burial, and resurrection of Jesus Christ, and understand the truth about God's chosen people in Christ Jesus.
Romans 11 is in answer to new Christians from among the nations, that believed no Jews could ever be saved, or as you said earlier, repent and believe unto salvation.
The early Christian converts, as with many today from among the nations, rightly held the Jews most responsible for Jesus' crucifixion. However, their prejudice to think they have all become reprobates from God, is of course false. It's false because it denies the NT gospel and doctrine of Christ, that no man is now accepted nor rejected by God through any kind of natural birth.
One of the reason many Jews of Jesus day, and today, hate Him so much is for declaring that with His coming, and then His death, there is no profit with God by natural seed and birth.
It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. (John 6)
They first despised the Baptist for saying much the same thing, about how God can raise up a bunch of rocks to be children of Abraham after the flesh.
In any case, Paul is reminding the new Christians that, as you say, the first repenters and believers were Jews, as well as the first chosen apostles of Jesus Christ.
Any soul can now repent of sinning and obey Jesus the Lord, and so be grafted into His green and holy olive tree of the house of Israel and Judah. First were the Jews by regrafting and then the nations by grafting for the first time. Although the foreshadowing of it to the nations was begun with such converts as Rahab and Ruth.
The green and holy olive tree and tabernacle of David, is now of course the body of Christ, that is now being built by Him from the foundation laid by His apostles.
After this I will return, and will build again the tabernacle of David, which is fallen down; and I will build again the ruins thereof, and I will set it up: That the residue of men might seek after the Lord, and all the Gentiles, upon whom my name is called, saith the Lord, who doeth all these things. (Acts 15)
Now, if you want to talk about a special promise of prophecy remaining for the natural Jews of the seed and stock of Abraham, then we can move on to
after the Lord's return, when He will give the land promised personally to Abraham and Hebrew and Jewish seed.
In that day will I raise up the tabernacle of David that is fallen, and close up the breaches thereof; and I will raise up his ruins, and I will build it as in the days of old: (Amos 9)
We see how the Lord quotes His prophecy differently from Amos 9 to Acts 15. He applied it to His church He is now building, without taking away from it's fulfillment after He comes again.
But, at this time, the only promises of God are to them that repent of sinning and believe His Son's gospel unto righteousness, whether Jew, Greek, Scythian, or any other nation on earth:
Grace and peace be multiplied unto you through the knowledge of God, and of Jesus our Lord, According as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue:
Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.