good1 you said there’s no mention of man speaking, but the scripture says “my word will not return to me void”, so who is returning his word to him. And I disagree with what you said about your words having no effect on what happen with God. In Luke chapter 1 when the angel told Zacharius he and his wife were going to have a son and he asked for a sign which was a sign of unbelief the angel shut his mouth until the child was born and he agreed his name would be John. And the children of Isreal when they spied out the Promised Land and the ten spies came back with an evil report and the congregation said they would die in the wilderness and this was after God promised them the land and GOD said “As ye have spoken in my ears so will I do unto you. What you say determines what you receive from God. The word of God is always good, but you can speak against it and not receive what God said.
In your example from Luke, Zacharius' faith had no bearing on whether or not the child, John the Baptist, would be born. Zacharius' muteness was a chastisement from God for his unbelief and had no effect on what God had previously determined to do.
On the issue of entering the promised land, the people received in themselves the consequence of their sin of unbelief. Their words reflected what they believed in their hearts.God's punishment for their sin would have happened if they had said nothing at all, just didn't believe they could enter and conquer the land.
The phrase, "will not return to Me empty" is the Hebrew way of saying, "What I have said I will do, I will do." In Isaiah 55:11, the illustration is the rain. It falls from heaven, it accomplishes the purpose for which God sent it, and it returns to heaven (v.10) without any activity, act, or spoken word, from man. It is the same way with God's word, which He speaks in absolute sovereignty, it goes forth and the word itself performs the tasks for which God sent it. It is just another way of saying that what God decrees to do, He will do.
Consider Isaiah 46:11, "Truly I have spoken; truly I will bring it to pass. I have planned it, surely I will do it." And what is God talking about? Verse 10 tells us, "My purpose will be established, and I will accomplish all My good pleasure." What any man may say, or not say, has nothing to do with it.
This world, and everything in it does not revolve around man. It revolves around God.