The problem is that acceptance of grace is in itself a grace. If man can't do anything without God's grace, then he can't accept the graces necessary for salvation without receiving a grace of acceptance of these graces.
That's not a problem.
Your problem is insisting upon confusing the grace that makes this freely-willed acceptance possible with God magically making a man's will no longer his own, believing man must turned into a puppet on a string by the grace to accept grace, yet still actually will something.
Receiving un-asked-for power to do something, even if that something is to exercise my will to make a particular choice, is not a violation of the freedom of my will in general or in that decision.
Therefore, saying "God condemns them for rejection of graces" is merely moving a problem one step back - if they rejected the graces, it means that God did not give them grace of acceptance of graces.
No, it doesn't. It just means they exercised their free will to refuse to cooperate.
Unless you want to say that man is able to accept graces by his own power, apart from God's grace, which leads to semi-Pelagianism.
You're just not getting it. Without God's grace, there's no grace to accept in the first place, leaving man lost, and without a prevenient grace, man's will, bogged down in sin, is not free to choose to accept it, and so has no way to salvation. That grace frees and empowers the will to be able choose God does not mean just its opposite.
Beliveing in man actually having a really real free will is not any kind of Pelagianism, regardless of your turn of phrase.
As I wrote numerous times here, no theological system accurately explains predestination, all of them taking to logical conclusion result in Calvinism or semi-Pelagianism.
Moot. We don't care. Even your idea of "semi-Pelagianism", straw man alternative that it is, has never been condemned by the Church, and we therefore have no reason to even reject it, as you've formulated it, as some would-be "semi-heresy".
We have to accept that predestination is a mystery which we cannot understand.
That doesn't resolve a contradiction. It leaves it there, and no appeal to a "mystery", that ubiquitous Latin copout, will ever make a logical contradiction possible. The proper response to a
reductio ad absurdum is to get rid of a false premise that led one to it, not to declare the question of how a contradiction is possible a mystery so you can just stick with it.