What is the significance of Exodus 4:10–16?

Immediately after convincing Moses of his ability to [somehow!] convince the elders of Israel to listen to him, Moses says, “O my Lord, I have never been eloquent…I am slow of speech and slow of tongue” (vs. 10). The Lord reminds him that he is the Creator and is therefore bigger than any speech impediment. He assures Moses that he “will be with [his] mouth and teach [him] what [he is] to speak” (vs. 11–12). Remarkably, Moses persists in his unbelief and asks the Lord to “please send someone else” (vs. 13), at which point the Lord gets angry (vs. 14). In frustration he asks Moses (!) if he’d feel better if his brother went along as his spokesman. He then once again reassures Moses, “I will be with your mouth and with his mouth” (vs. 15).

If it was foreknown from all eternity that Aaron would be Moses’ partner, the Lord’s attempt to convince Moses otherwise and the Lord’s subsequent frustration with Moses becomes inexplicable. How can one sincerely try to accomplish something they are eternally certain won’t be accomplished? And how can one truly get frustrated over something they were eternally certain would happen?