That the Torah is everywhere and in everything. In particular  the letters of the Torah are in everything. Not that everything is Divine. But that there is Divinity in things. Even in Evil there is divinity. For without God making everything exist nothing could exist. But the Divinity in Evil is hidden. But if it would be open Torah inside of Evil the the Dark Side would have nourishment from open Torah. So instead in evil is hidden Torah.
The Torah as a whole is in principle contained in the Ten Commandments. These were hidden in the world until they were revealed at Mount Sinai. These commandments are thus contained in a hidden way in the ten statements of creation. The hidden Torah inside of evil is in the first hidden statement of creation, the מאמר הסתום. So one can find God even if he has fallen  from holiness. One can return to God even from the lowest depths of Hell.

[IV] Not to be stubborn about anything. I think this is related to his idea that one should learn Torah fast. We know that when you lean fast there remain things you do not understand. But He thought going back and reviewing these things is not necessary and not even desirable. What you got, you got. What you did not get, .. don't worry about it.

[V] The Will. I have not be looking at Philosophy for a long time so I can't write much about this. But mainly we know about the Dinge An Sich of Kant. The thing in itself. There are two aspects of this. Material things and ideas. It is complicated. But Schopenhauer took this idea in a direction different than Kant. He though there is only one Thing in Itself, the Dinge An Sich. The Will. And the whole world is just a representation of that Will.