What is depersonalization and derealization disorder from a neurological point of view? Lately, there is a lot of information on the psychological, experiential and behavioral aspects of DP/DR. 

(I find it most interesting as many have described it as having one foot in the spiritual realm and the other in this.)

When is it defined as an experience and what is the criteria that makes it a disorder?

As a disorder how is DP/DR described from a scientific neurological point of view?

Is some part of the brain damaged?

How is it physically similar to other disorders or issues (stroke, epilepsy)?

What methods directly or indirectly reverse the changes caused by it?

Why is this considered a relatively "new disorder" as I am sure it has been experienced in past decades?

Is it possible many have it and don't know it, as it appears there is a lot of subjectivity in defining it?

Any information is greatly appreciated.
You ask for a very in detail analysis of depersonalization (DP) and derealization (DR) disorder. I'll try to meet your expectation. 

Lets start with some definitions. Depersonalization and derealization happen after people have experienced great amounts of psychological stress or brain injury. The likes of which their coping measures fail to handle. The brain in an effort to contain the pain cuts off some contact with reality or self identity. DP/DR can be a sign of the onset of psychosis in bipolar or schizo type individuals; however it is not correlary to psychosis.  "it was like a dream" is how someone speaks of derealization and "did that happen to me or did I hear about it happening to someone else" is depresonalization. 

DP/DR (not the disorder) is very common among those who have been in any way traumatized (especially head trauma) happening about half the time. (Hunter 2004) Basically when you hear on TV "He's going into schock". They mean the patient is experiencing an acute stress reaction one of which features is DP/DR. 

Because DP/DR is oft encompassed by shock let me describe the physiological symptoms and treatment of shock before we discuss the psychological and neurological signs. 


Appear as a living zombie. 
Cool, clammy skin which may be pale or gray. Ie the blood drained out of their face.
Abnormally weak or rapid pules and breathing. 
Low blood pressure.
Absent staring eyes with occasional pupil dilation. 
Sometimes overly excited or catatonic


In response to shock always get professional medical help by dialing the emergency number of your town/nation like 911. Make the person as comfortable and still as possible with reassurances until professional help arrives. Do not give them any food or water as the body is unprepared to handle any additional input. 

Shock happens before DP/DR disorder. When the specific features of DP/DR continue or reemerge longstanding after trauma it becomes a disorder. (Simeon 2006)  As a disorder it is the third most frequent. (Simeon 2004) A self administered intake survey or an hour long psychological interview following DSM guidelines allows a doctor to make the diagnosis. 

The general guidelines provided by the DSM-IV-TR: 


  Longstanding or recurring feelings of being detached from one's mental processes or body, as if one is observing them from the outside or in a dream.
  
  Reality testing is unimpaired during depersonalization
  Depersonalization causes significant difficulties or distress at work, or social and other important areas of life functioning.
  
  Depersonalization does not only occur while the individual is experiencing another mental disorder, and is not associated with substance use or a medical illness.
  
  The DSM-IV-TR specifically recognizes three possible additional features of depersonalization disorder:
  
  Derealization, experiencing the external world as strange or unreal.
  Macropsia or micropsia, an alteration in the perception of object size or shape.
  
  A sense that other people seem unfamiliar or mechanical.


-Wikipedia Depersonalization Disorder

Your first assertion that DP/DR is associated with spirituality is correct. As when psychotic hallucinations happen they typically have a DP/DR feature to them. It is common for something obviously psychotic like seeing an angel or devil to be associated with an out of body experience or vision type trance. Common is it for someone to experience a trip to the spirit world to see a loved one. Such events are to a limited extent drug induceable. (I do not like many other students of psychology believe that all spirituality is aberrant however much is caused by illness.) Psychologist typically teach a patient what is a hallucination and psychosis and allow the individual to label their experience as either spiritual or psychotic. 

Now the neurological basis is diverse as it is not necessarily an organic originating condition but a response disease. I was able to find two main theories first a functional difference and the second based on lesions in the brain. 


  On the basis of this, a new model is proposed according to which the state of increased alertness observed in depersonalization results from an activation of prefrontal attentional systems (right dorsolateral prefrontal cortex) and reciprocal inhibition of the anterior cingulate, leading to experiences of “mind emptiness” and “indifference to pain” often seen in depersonalization. On the other hand, a left-sided prefrontal mechanism would inhibit the amygdala resulting in dampened autonomic output, hypoemotionality, and lack of emotional coloring that would, in turn, be reported as feelings of “unreality or detachment.”


-Depersonalization: neurobiological perspectives


  Although DP/DR can occur as a primary condition, it often accompanies other psychiatric  or neurological diseases.
  
  Most of the neurological conditions associated with DP/DR, such as epilepsy, migraine, and mild head trauma etc have poor localisation value. However, DP/DR have also been described accompanying localised lesions. This raises the question if the study of such cases (the lesion method) might be a valuable approach to study the neurobiology of DP/DR.


-Separating depersonalisation and derealisation: the relevance of the “lesion method”


  Depersonalization appears to be associated with functional abnormalities along sequential hierarchical areas, secondary and cross-modal, of the sensory cortex (visual, auditory, and somatosensory), as well as areas responsible for an integrated body schema. These findings are in good agreement with the phenomenological conceptualization of depersonalization as a dissociation of perceptions as well as with the subjective symptoms of depersonalization disorder.


-Feeling Unreal: A PET Study of Depersonalization Disorder

To answer another one of your questions as the above papers show it may or may not be caused by organic brain damage. It is co-morbid to epilepsy but is its own distinct diseases. That is to say epilepsy is caused by overloads in neural pathways like a neural thunderstorm. DR/DP is what is left over after the storm starts destroying pathways. Likewise with oxygen deprivation and a stroke. 

There are no drugs to treat all the symptoms of DR/DP instead antidepressants like Prozac and anti-anxiety like Klonopin are known to help sometimes. 

Freud created a very basic and useful treatment for DP/DR called Reality testing which is a main component in CBT. In it you logically examine and reflect on your emotional responses and see how you correctly or incorrectly responded. The you make adjustments to become more rational. Rather than fixing the underlying problem it allows the sufferer to adapt to the disease. 

DR/DP is not always associated with a delusion. So sometimes people can self identify the onset of an episode.  

All things psychological are new but DP/DR is not very new. It was introduced in DSM-II in 1968. Yes DP/DR diagnostic is controversial in the psychological science. This is because it is used in criminal law. As it is a major question if someone who commits a crime in a DP/DR state should be held equally accountable. (Saks 2000)

References 

Hunter EC, Sierra M, David AS (2004). "The epidemiology of depersonalization and derealisation. A systematic review". Social psychiatry and psychiatric epidemiology 39 (1): 9–18. doi:10.1007/s00127-004-0701-4. PMID 15022041

Simeon, D., &amp; Abugel, J. (2006). Feeling Unreal: Depersonalization Disorder and the Loss of the Self. New York, NY: Oxford University Press.

Simeon D (2004). "Depersonalisation Disorder: A Contemporary Overview". CNS Drugs 18 (6): 343–54. doi:10.2165/00023210-200418060-00002. PMID 15089102.

Sierra, M., &amp; Berrios, G. E. (1998). Depersonalization: neurobiological perspectives. Biological Psychiatry, 44(9), 898-908.

Sierra, M., Lopera, F., Lambert, M. V., Phillips, M. L., &amp; David, A. S. (2002). Separating depersonalisation and derealisation: the relevance of the “lesion method”. Journal of Neurology, Neurosurgery &amp; Psychiatry, 72(4), 530-532.

Simeon, D., Guralnik, O., Hazlett, E. A., Spiegel-Cohen, J., Hollander, E., &amp; Buchsbaum, M. S. (2000). Feeling unreal: a PET study of depersonalization disorder. American Journal of Psychiatry, 157(11), 1782-1788.

Saks, E. R., &amp; Behnke, S. H. (2000). Jekyll on trial: Multiple personality disorder and criminal law. NYU Press.