Unfortunately, history shows that the likelihood of making converts amongst an enemy tribe is far smaller than a remnant of that enemy within one's ranks influencing your culture and thinking.
I notice that you have missed answering the bulk of my post in this part.
What is wrong with the methods that I outlined?
So, you would like God to intervene in every aspect of life and in every instance?
How you get that from my post is a mystery.
I highlighted an intervention by God. In this case commanding genocide. I am asking why he couldn't have achieved his end of protecting the Jewish tribe by less extreme means.
It seems easy to imagine ways in which an omnipotent God could achieve this.
You want him to make people into robots who don't think?
I can't see how the methods I outlined makes people into robots.
However, he seems to want to make people into dead people who don't think
It's also worth remembering that for each of the instances of 'genocide' there are several instances of his employing alternative solutions.
Giving examples of where you have not committed genocide is not an excuse for the times when you did.
There is a difference between 'erring' and 'sinning'.
"Therefore, there remains the possibility that God can sin.
Hence the original question. How do you know God can't/doesn't sin?"
Happy now?
For instance, I can take a wrong turning whilst driving to visit friends or family that might take me miles out of the way. Have I sinned? It would depend on whether I did it intentionally. I could, as a civilian, wound or even kill an intruder who threatens the life of me and my family. Have I sinned? In the latter example, I might have broken the law, but have I sinned in the eyes of God?
In the eyes of the law it would depend on whether or not you could reasonably have used less extreme means.
Going on to wipe out the perpetrator's extended family/tribe so they can't do it again is definitely going to be seen as extreme I think.
All I am doing here is asking the same questions of God.