I don't think wanting to get revenge for Saddam trying to kill his father was a significant factor in his decision to invade Iraq. Any serious President (that is, excluding people like Clinton, Obama or Biden) weighs seriously the consequences of sending troops into harm's way.
Unfortunately I don't think GWB will get over Trump humiliating his brother in 2016, but he has zero political power and virtually no political influence at this point, so it doesn't really matter (other than hurting his nephew's political fortunes).
The case for going to war was regularly posted here, and it was overwhelming. Saddam broke the cease fire. We were absolutely justified in defending ourselves. We couldn't just ignore him.
Roberts is interesting. When all else is equal he is definitely a conservative justice. Unfortunately, he seems to have a strong aversion to using the court to overturn the status quo. That's why he was on the right side of Obergefell but the wrong side of the Obamacare decision. I don't agree with him, and it's disappointing.
Put yourself in Bush's shoes. You see a guy who was a Democrat during your presidency, who you know was lying about you and trying to undermine our war efforts. A guy who praised Pelosi and wanted you impeached. Now you see him switch parties to run for president. He gets on the primary stage and he's attacking you, still spewing lies about you, and using those lies to attack your brother. Hell, he even said you allowed 9/11 to happen. Then you watch as your party nominates the liar. Wouldn't you feel like the voters of your own party had stabbed you in the back? I would. Is he supposed to kiss Trump's ass or something? If it happened to Trump, he'd be openly supporting the Democrats, there's no doubt about it. Especially since he's a political carpetbagger.