Why would Zelenskyy use long range missiles inside Russia? All of European Russia(and most of Siberia) is within range of intermediate range missiles fired from Ukraine. Yes, parts are Siberia are beyond 5,500 kilometers away, but only a tiny part of Eastern Siberia. Why bother with using long range missiles?
Why use American missiles? Ukraine has an aerospace industry from the Soviet Union. While it has been allowed to rot recently, it is still decades, if not centuries, ahead of the Houthis. They could produce their own missiles using easily available parts.
Your hypothetical makes no sense. Let's talk about a real possibility. If the Ukrainians use intermediate range missiles produced in Ukraine, with parts bought on the open market, to hit targets in Russia, what then? I would assume that Russia would continue to fire missiles at Ukraine, though they might expand the war. Ukraine has already fired drones, and cruise missiles into Moscow, so there is precedent for saying things would not get worse.