Basic definitions, Robert. You use the word "unjust" and it appears you think it simply means "unfair". Am I right? If not, what does it mean to be "just" (or "unjust")? The definition of the term is pretty straightforward, I think. It is where we get our word "justice" from. It isn't about being fair or unfair, it is about doing what is right - what is according to truth. That is a far cry from "fair".
Remember what He did in Noah's day? That was justice. Remember what He did to Israel in response to their idolatry? They were sent into captivity. That was justice. He even told them ahead of time what would happen. That is justice. And did He serve justice as soon as they strayed? No. Far from it. He tolerated wandering after wandering - even after He gave them a generation in the wilderness to show them what was in their hearts (Deuteronomy 8:2). And at some point, He finally gave them justice. But not after years of tolerance and forbearance. Is that justice? No, that's mercy. Read Psalm 78 and see how just God is. The heart of that chapter reads like this :
But he, being full of compassion, forgave their iniquity, and destroyed them not: yea, many a time turned he his anger away, and did not stir up all his wrath.
Psalm 78:38
Yet, of the same people, God directed this :
And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not.
Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.
Isaiah 6:9-10
God is telling Isaiah to prophesy so that Israel is hardened. How is that just? It certainly doesn't seem fair, but I would say it is just. These people have continually rejected God. Over and over again gone back to their own ways and rejected their Savior. So God is giving them over to their wickedness. Even a generation that may not have had the chance to do right or wrong - they are being judged for the sins of the fathers. How is that fair? How is that just? They may hear the words of warning, but God has determined that it will be to their destruction, not to their repentance and salvation.
Jeremiah sums up the problem very succinctly :
The heart is deceitful above all things, and desperately wicked: who can know it?
Jeremiah 17:9
And Jesus echoes this sentiment :
Ye have heard that it was said of them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
Matthew 5:21-22
...and...
Ye have heard that it was said by them of old time, Thou shalt not commit adultery:
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
Matthew 5:27-30
We have the evidence of thousands of years to show that there is none righteous (no not one) and that there is no excuse. The problem is not with our actions, it is with who we are. That's why God (who is omniscient) can say that He loved Jacob but hated Esau...even before the two were born. He wasn't forseeing what they would do, He was making a Sovereign decree. Jacob was a rotter and God loved Him. Esau wasn't the manipulator Jacob was yet God still hated him. Since none are justified before God by their own works, none can call God unjust for whatever He does with His Creation.
Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will?
Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus?
Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
Romans 9:19-21
It seems to me you are (repeatedly) trying to advance the very argument Paul was putting down in Romans 9. But your OP isn't even really consistent. You have railed against Calvinism (supposedly) having God as unjust, but you have clearly used a different definition for the term. Used properly, there is no issue with the Dordt statement. One doesn't need to be Calvinist to agree with that, either.