1. Ok, i understand thats not easy to fix and bonus is not really strong, so it doesnt worth an effort to fix.
2. As a temporal fix disabling dual classing from paladin/sorcerer is ok solution. But i really think its better to move the autocast if possible.
The Tactical Autocast adds +1 Cast Time, but you could select Effortless Casting (Sorcerer), Savant (Mage), or Nimble (Sovereign Talent) and it would nullify that penalty;
And when dualclassing from Sorcerer to Mage you can take Effortless Casting and Savant and loose Sorcerer penalty resulting in -2 casting time = instant blizzard.
3.Well, the one thing AI is really bad at is dealing with doomstacks. If doomstack speed is 5-6 there is a chance for AI to win just by pure numbers of its superior units, but a doomstack with speed of 11 just annihilates entire nations in a single turn... Hmm, this problem is probably amplified by point 4 a lot. Without Surging there would be no 6 level 30+ heroes
4.No, its really broken. I just opened my current game, my mage (in an army with 3 henchmen) has a total XP bonus of 270%, my lone sovereign paladin has 380%. And i made no effort to specifically abuse Surging, i only taken that trait line on combat heroes. If i had taken it on my 10 governors...
5. I personally altered the XtraSavage AIStartingFundsRatio to increments of 10 (eg 10 for XtraSavage1, 100 for XtraSavage10). This feels much better. I think you really underestimate the effects of starting gildars on AI - with your settings Insane feels much more difficult than XtraSavage5.
6.The "blizzard or gtfo" is not so about nerfing blizzard but about upping other spells. Currently the only reason to cast anything else is when you are fighting a single singleentity unit or when enemy is resistant to cold.
7. The only problem i have with Quick March is a description. I was like 'WTF it does?!" until i looked in xml.