In the instance with Moses, He wasn't.
Yes, God "changed His mind," knowing from before the universe existed that He would. This is how we have to speak about a God who is far more unlike us than like us, who has always known everything, is in every place in the universe simultaneously, and can do anything that it is logically-possible, and in keeping with His essential nature, to do. Inevitably, we end up talking about Him from our own frame of reference, using phrases and terms that we understand but that actually only approximate the truth about Him. I think this is the case with God "changing His mind."
It doesn't matter what those who escaped God's wrath thought about Moses' idea. God knows and has always known what's best. All His ways are right and true. To say otherwise is to diminish God and perhaps to think of God in a way that is false.
Yes. But he was, as a result of his disobedience, kept from entering the Promised Land, remember. He was not a perfect man; he made mistakes - even though he was chosen by God to lead Israel out of Egypt.