1) VR is going to have to advance a hell of a lot before it replaces certain aspects of legit hands on weapons training. Infantry guys train in "glass houses" and I can easily see at some point down the road when the tech has advanced enough that VR might become a part of standard training doctrine. They'd need to figure out a way to incorporate handling real weapons into the VR software, something along the lines of building the software around MILES gear. Nothing will ever replaces certain aspects of weapons training though as a lot of my units weapons training was things like reloading drills which the sole point is developing muscle memory.
2) Read the DSM. It's really that simple. The DSM is the book of diagnosis criteria for mental health related issues, and the symptoms are well documented. You're not going to find a hidden component because there isn't one. The only "hidden" thing you'll find is people struggling to hide their symptoms from others, and a lot of the time failing.
3) This is inherently complex. Speaking VERY generally you have three classes of soldiers. You have the "lower enlisted," which are the soldiers with none to very little real responsibility. Think Privates. Next you have non-commissioned officers, or simply NCOs. Think Sergeants. Then you have commissioned officers which run the gamut from Lieutenant up to General. You will very rarely see soldiers of these three levels of hierarchy interacting socially, and at times even professionally. For example, you will almost never have a brand new lower enlisted Private talk to the Company Commander (a commissioned officer, usually a Captain) unless directly ordered to do so. There are a ridiculous amount of exceptions to all of this though, and it's not something that can be easily explained to a layman. You're going to boil this question down to more specific questions.
4) Imagine you're married. You obviously care for your significant other. You know them intimately, all those stupid individual quirks you didn't discover until they'd become completely relaxed around you and showed you who they really were as a person. Now consider that most soldiers spend more time with those they serve with than their own wives/husbands and family. You come to know each other very well. You learn their quirks. You may not like each and every one of them, but if someone outside of your platoon fucks with anyone within it then it doesn't matter if you like them or not because they're for all intents and purposes your family, and nobody fucks with your family. Then you go of to war, and this guy that you've come to know so well, this guy that is family to you, this guy that you know better than your own wife, he gets blown up by an IED. He's missing a leg. The rest of him is burnt. He's suffering. He's dying slowly, and there's not much that doc can do for him. You can't help him because you're too busy fighting back and you'd only get in docs way anyway. But as you're shooting and being shot at you know that this guy that you care about so much about is in agony and you're powerless to do anything about it but try to kill the fucker that did this to him. If you have no frame of reference for what this might feel like, then I don't know what can help you.