Dehumanization results from the soldiers' lack of hand-to-hand combat. The shell warfare creates a bizarre non-entity rather than an enemy soldier. Because Paul and his fellow soldiers strive to avoid being shelled, they crawl on their bellies in the dirt and mud. In Chapter Four, Paul narrates,
We march up, moody or good-tempered soldiers--we reach the zone where the front begins and become on the instant human animals.
In Chapter Six, after witnessing the horrors of warfare, Paul feels disconnected from his past:
And even if these scenes of our youth were given back to us we would hardly know what to do. The tender, secret influence that passed from them into us could not rise again....
In this same chapter, Paul expresses his alienation:
We are forlorn like children, and experienced like old men, we are crude and sorrowful and superficial--I believe we are lost.
#3, may I grab a snippet of your quote?
Yesterday we were under fire, to-day we act the fool and go foraging through the countryside, to-morrow we go up to the trenches again. We forget nothing really.
As films like M*A*S*H have shown us, soldiers develop a certain mindset to help cope with the horrors of war. A sick or bleak sense of humor becomes essential if one is to remain sane. In this case, you are correct that the quote shows disconnectedness, but it also shows dehumanization in the sense that the war changes the very essence of who these people are. They "forget nothing" and yet they forget how they lived before the war, thinking of "foraging" as "acting the fool." Civilians would never consider foraging anything but a chore; the soldiers see it as a welcome diversion, almost entertainment.
Consider the significance of the following quote from Chapter 7:
I feel awkward. The suit is rather tight and short, I have grown in the army. Collar and tie give me some trouble.
There is deep significance in this. Not only does Paul find it difficult to live a normal civilised life with a suit on, but also he has grown. This growth of course does not just refer to his physical growth, but also how he has changed through the experiences of fighting in the war. In so many ways, this growth makes him alienated from the society he is fighting to protect.
Here is a quotation that deals with a sense of alienation and dislocation. The opportunity to return home from the front is not what he had expected. Instead, he now feels alienated and dislocated from home.
I imagined leave would be different from this. Indeed, it was different a year ago. It is I of course that have changed in the interval. There lies a gulf between that time and to-day. At that time I still knew nothing about the war, we had only been in quiet sectors. But now I see that I have been crushed without knowing it. I find I do not belong here any more, it is a foreign world.
In my book, this quote is on the first page of chapter 7.
Habit is the explanation of why we seem to forget things so quickly. Yesterday we were under fire, to-day we act the fool and go foraging through the countryside, to-morrow we go up to the trenches again. We forget nothing really. But so long as we have to stay here in the field, the front-line days, when they are past, sink down in us like a stone; they are too grievous for us to be able to reflect on them at once. If we did that, we should have been destroyed long ago. I soon found out this much:--terror can be endured so long as a man simply ducks;--but it kills, if a man thinks about it.
I chose this quote to indicate how they are coping with war. I think it is a perfect description of disconnectedness, as well as pretty much all of your terms except the last.
With respect to dehumanization, I would offer this quote:
We want to live at any price; so we cannot burden ourselves with feelings which, though they may be ornamental enough in peacetime, would be out of place here.
This shows that the soldiers at the front have become very different from normal people. They know that feelings can only be a bother to them and so they get rid of them. To rid yourself of all feelings is certainly a dehumanizing thing.