Thank you for the work that you did in gathering this information. It is a compelling story, and one that I will certainly save and study more carefully.
The question of salvation for children will probably continue right up to the time of Christ's return. I will admit that we all see through glass dimly, especially on this issue. It is natural for man to look at a young child and see nothing but innocence, but we do not see as God sees.
Although I will not address each reference individually, many of your examples if not all, are based on the children of His chosen people. Take Deut 1:39 for example. God is punishing His people for loosing faith. However, He did not include the children. They would inherit the land as God promised, maintaining a remnant of God's people.
You also had a reference to Rom 9:11. Let's expand this a bit to be Rom 9:10-13.
10 And not only this, but when Rebecca also had conceived by one man, even by our father Isaac 11 (for the children not yet being born, nor having done any good or evil, that the purpose of God according to election might stand, not of works but of Him who calls), 12 it was said to her, “The older shall serve the younger.” 13 As it is written, “Jacob I have loved, but Esau I have hated.” Now ask yourself – If it had been God's plan for Esau to die at the age of 6 weeks, would he have been saved or was he chosen for destruction prior to birth?
It is for reasons like this – that I cannot go along with Age-of-accountability. It is either true all the time, or age is not a factor in God's decision. I prefer to believe that God has always planned to have a people who belong to Him, and He will always preserve a remnant to accomplish this.
The Canons of Dort makes the following statement which I believe to be true:
Since we must make judgments about God’s will from his Word, which testifies that the children of believers are holy, not by nature but by virtue of the gracious covenant in which they together with their parents are included, godly parents ought not to doubt the election and salvation of their children whom God calls out of this life in infancy.
Granted this is not the word of God, and was included to be a comfort to His people upon losing a child. However, since God will maintain His remnant, it stands to reason that it will be done through His people.