In Acts 7:51, Stephen is speaking to the Jewish leaders, the same people who refused to believe Jesus was the Messiah, and who continue to stubbornly refuse to believe it after His death. While God chooses whom He will save, He also, correctly, reminds them of their responsibility in their refusal to believe. Their inability due to the fall. Their rebellion.
In Hebrews 3, the writer is contrasting the New Covenant with the Law of Moses, how the New covenant is a superior covenant than the law and the different types of responses they draw from people. In OT times, they did not have the Holy Spirit to guide them, but now they do. But because they are addressing a Jewish audience who has lon believed the law was the path to salvation, how they address the matter is different.
Damnation IS our fault, no matter what. Adam, as our human representative before God, chose sin and rebellion, directly causing the fall and our damnation. We are ALL under that curse. God, in His mercy, chooses to save some. It is only because of His mercy and grace that we will get to spend eternity with Him. Not because of anything we have done.
And speaking to the issue of decisions to be made, the problem with your assertion is that many are saved against their desire to have nothing to do with Jesus or God. Saul/Paul is one example. And if you read many current personal testimonies, you will see the same phenomenon. My own testimony is of that nature. So, while Calvinism can include the slow, soul searching, studying for the truth type of conversions that one may think they have a hand in, your free-will belief does not encompass all those who have a “road to Damascus”-type experience.