Why did Iblis say: "Because You have sent me astray, I will surely sit in wait for them on Your straight path," even though he was the one who refused to prostrate to Adam? And does Allah lead astray or guide? And is this verse a dialogue between Iblis and his Lord?

"Ighwaa'" (misguidance/allurement) means leading astray and pushing away. Indeed, Iblis said: "Because You have sent me astray [aghwaytani]" (Quran 7:16), meaning, because of what You have put in his heart of error, obstinacy, and arrogance. Iblis knows that misguidance is in the hand of Allah Almighty, just as guidance is in His hand. Allah guides whom He wills out of grace and misguides whom He wills out of justice, and He misguides none but the defiantly disobedient (al-fasiqin). And Iblis was defiantly disobedient, so he deserved misguidance and being led astray. It is from the belief of Ahlus-Sunnah wal-Jama'ah that Allah guides and misguides.