He would've had to. You can't trigger the High Priest ending without having the shirt, unless I have a horrendous memory.
Otherwise you have to settle with the Chaos/Noble ending (and thus lower score multiplier)

Nope, so far as I know there's nothing special about that bookcase. Of course if you don't have a pickaxe you can always chuck some acid on it or kick it down if your legs are strong enough.

As for the coffin in the catacombs, we ran some tests a while back and found that placing or removing an item from it will randomly boost your alignment in either direction. So it's not very useful, but interesting nonetheless. Could be added to later.

And as for Valpurus well... He's basically THE God. He's literally holding the world up on his back.
Imagine the most pompous, arrogant, self-righteous person you can. Now multiply those qualities by 100. You now have Valpurus.
He very much considers anyone but the most pious of worshippers as a blight and gets VERY pissed if you try to waste his time by praying to him otherwise - he's got better things to do like hold the world up, ya know? He's a very old, very grumpy old frog. Silencing a single mortal who is bothering him is pretty much him doing the world a favor in his eyes.

Also we must remember that this is IVAN and as such "lawful" pretty much boils down to "doesn't go out of his way to harm humans without provocation" rather than being a force of love, peace and order.

Congratulations! I see you've already added your win to the honor rolls.
Second highest score for vanilla IVAN too! Not too shabby.

This seems about right. He's the god of Atheism after all.

Could've sworn it only turns some of the town hostile when you present the shirt rather than every man and his dog, I shall have to go check.
EDIT: Huh. Guess I was wrong. But yes the town goes back to normal as soon as Petrus bites the dust so I suppose that makes it slightly easier to pull off than the chaos ending.

Do note you have to grab the Shirt of the Golden Eagle before you kill Petrus if you want the High Priest ending. Otherwise you can just skip out on even going to Oree at all and just kill Petrus and take his nut out of town.
The only difference here when going for the nut ending is that having the shirt prevents the entire town from attacking you when you challenge Petrus.

Work a little through Gloomy Cave before you go there and stock up some wands and other equipment (namely you should be able to find or enchant better armor and weapons). If you can't beat the eagle in an honest fight then just blast the thing with wands of striking/fireballs/lightning until it is dead.
IVAN's never been about every victory being an honest one

Also not much to gain from Decos once you get to the point where you can beat him in a fair fight, hence why I consider him a target of opportunity at the start of the game but not worth it later.
Say, if you were to encounter gas grenades or the holy hand grenade.

It's worth noting that although, yes, they will hit like a truck full of lead cinderblocks, metal and rock golems have very low to-hit values. So, it's somewhat safe to take them on if you have high AGI - just don't expect to kill them without something REALLY strong or explosive.

It's entirely possible it trains END in some way. Hell, IVAN's stat training is so "detailed" that I once levelled my DEX by putting a ring on.

Also on the subject of getting hesitant, I get this way before the floor with the guaranteed mystic dark frogs. HORRIBLE things.