Well first, what you’re asking a theist to defend is a premise that we must first embrace from you; and it is frankly a premise that I do not embrace, nor do the majority of theists I know, because we have no reason to. You’ve even admitted that you don’t have substantiation — that this is just your belief, springing from what you claim are a number of reasons. The fact that carbon is abundant has no bearing on whether God, if he exists, has chosen to create life elsewhere of which we are unaware. He is absolutely capable of creating a carbon-rich universe where just one single human exists.
And this is where I will leave your third question: you’re asking a question that allows for two things — your belief, and God’s plan of salvation for mankind through Jesus. If both are true, then I have good reasons to believe that God is a God who would know the right way to handle intelligent life in other galaxies. If they are self-aware and sentient, as we are, and they are sinful, God by his nature would either judge them or have a plan of salvation as well, but he is not bound to have a plan of salvation. He is almighty and sovereign God in your scenario, and so he can do as he sees right and good.