The Jedi hadn't faced a true Dark Lord of the Sith in a thousand years (Maul being an apprentice and Dooku more of a fallen Jedi). On top of this, they were going against the most powerful Sith in history. Ep III novel states that the Sith had evolved over the 1000 year gap while the Jedi maintained complacency in terms of their philosophies and teachings. The novel even states that Yoda acknowledged that he couldn't defeat what the Sith had become. Yes maybe Mace and co were a little arrogant but I think saying they were ignorant more describes the situation.
Correct. The Jedi, if any of them thought at all about the reemergence of the Sith, expected something more along the lines of the war that ended with the detonation of the Thought Bomb. The Sith had done things in one particular manner for so long (like our Sith here in TOR, for that matter) that the Jedi just never saw the Banite Sith and their slow undermining of the Republic coming. And undermining the Republic wasn't even the point of the Plan. It was just a necessary step towards the true goal, which was the destruction of the Jedi. The Jedi thought the war was about control of the galaxy, but they were blinded by both the "shroud of the Dark Side" and their own expectations and flawed understanding of the goal of the Sith.