The prophecy written towards the disbelief of the Jews.
Isaiah 44:18
He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.
I'm not sure I understand the specifics of what you are aiming for or why your OP question stands in the Discipleship forum, but perhaps the following might help.
You cite the LORD's words to Isaiah at the latter's call to be a prophet in ch. 6:9-10 which in turn is used in John 12:40. Furthermore, the book of Isaiah is filled with varied thematic repetitions and allusions; as an illustration, 44:18 clearly recalls 6:9-10. There are of course some verbal variations between Isaiah 6:9-10 and John 12:40, but the borrowing source is clear (Isaiah in John).
God calls Isaiah to prophesy so that the people's (Judah's) hearts may become calloused, ears dull and so on. The verb phrase "make ... dull" in Hebrew is the same as used of what God did to Pharaoh's heart. God hardened Pharaoh's heart. Similarly God called Isaiah to prophesy so that Judah may become hardened to God's truth lest they "be converted, and I should heal them" (cf. 6:10 and your own citation). Put otherwise in allusion to 44:18, God called Isaiah to prophesy so that the people of God might turn to idolatry.
Isaiah was to prepare Judah (or the Southern Kingdom) for the judgment of exile (which occurred in the 6th century B.C.). Of course much of the book of Isaiah is rich with the grace of God and messianic prophecy, so presumably even those topics are what God used to harden the people's hearts. God hardened Israelite/Judah's hearts via His prophet Isaiah even using the truth and gracious words (cf. what Jesus says in John 8:45--"because I tell the truth, you do not believe me").
So centuries later, John records how the situation among God's people is similar to pre-exilic Judah ("the disbelief of the Jews"). The Jews by and large were rejecting their own Messiah Jesus (prophesied incidentally by Isaiah). Antagonism was building during Jesus' public ministry against Jesus, all aiming toward Jesus' crucifixion and a judgment against the nation in the siege and sack of Jerusalem, A.D. 66-70, which Jesus predicts with solemnity and sadness.
Of course Jesus' crucifixion and resurrection (etc.) led to the salvation of Gentiles far and wide (and to the salvation of some Jews like Paul and Peter). Question then arises whether God's promises to Abraham and the patriarchs has failed (Rom. 9:6). Paul says "no" and that topic is what much of Rom. 9-11 is about. God has always hardened whom He wills and had mercy on whom He wills (Rom. 9:18 etc.).
I often wonder if I am observing a like set of phenomena today among the people of my own larger family and nation. As time goes on in my own life time and somewhat before, fewer and fewer people of my family and nation are becoming Christians. That is not true the world over, but it seems true in my arena. I grieve over the progressive losses, but there seems nothing new in it to God.
Also note that my family and nation willingly reject the gospel once more widely respected (proportionate to the population of the given day). God hardens hearts and people harden their own hearts.
And surely here we find reason both to fear God and to worship Him!
"Oh, the depth of the riches and wisdom and knowledge of God! How unsearchable are his judgments and how inscrutable his ways!
“For who has known the mind of the Lord,
or who has been his counselor?”
“Or who has given a gift to him
that he might be repaid?”
For from him and through him and to him are all things. To him be glory forever. Amen." Rom. 11:33-36
