Are Amos 9 and Isaiah 11 the only Old Testament passages about the earthly Kingdom?
No, Amos 9 and Isaiah 11 are not the only Old Testament passages about the earthly Kingdom. They are just a few highlights, and indeed there are clearly many, many more references to this earthly Kingdom throughout the Old Testament.
Les explains that Isaiah 11 describes the earthly Kingdom where believers from Matthew 25 enter and become parents of children. In that Kingdom there will be no death, no sickness, no curse, no Satan, and Christ will be in total control, a perfectly perfect rule and government, resulting in a tremendous population explosion. Isaiah 11:9 says they will not hurt or destroy in all His holy mountain, because a mountain in the Old Testament especially is a Kingdom, and the earth shall be full of the knowledge of the Lord as the water covers the sea. Then Les skips through the Old Testament to Amos chapter 9, saying he is just hitting a few of the highlights, and there are of course many, many more references to this earthly Kingdom. So Amos 9 and Isaiah 11 are not the only passages; they are only a few highlights among many more.