"You're arguing from a conditional statement."
So what? What's your point?
It says, right there in Jeremiah 18, that God can say one thing (i.e. He WILL destroy a nation), but do another IF they repent!
"If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them."
How can God NOT DO WHAT HE PREVIOUSLY PRONOUNCED in Calvinistic theology?
It's simple. He can pronounce it knowing full well ahead of time whether or not it will be heeded.
The statement "If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them" remains valid regardless of whether or not God has a hand in their turning.