why would God hardened the heart of Pharaoh
I know this is going to sound crazy, but it's because YHWH had lost to Pharaoh in a game of Monopoly 2 weeks prior (Pharaoh scored Mayfair early on and the game was pretty much over after that. God lost $50 and a case of Stella, and was not a happy camper). Basically the whole 'Heart Hardening' incident was God just pranking Pharaoh, trying to get his own back. In the Bible there are plenty of examples of God's wicked sense of humour - take the Bear and the 42 children. So YHWH's prank was pretty run-of-the-mill, to be honest.
This whole theory is evidenced by how YHWH directly interacts with his home-boy Moses. God got Moses involved, and although he was a cry-baby at first, he was really into it by the 5th plague. Moses was just a 'front' so Pharaoh didn't catch on during the early stages.