Evidence for plurality outside of DID is NOT hard to come across.
The existence of multiple selves in normality, spirtuality and also in other disorders that arent just DID are pretty well studied. Here are a few sources that I found just today and im sure by reading these texts yourself and by following up on the references cited within them, you can find even more evidence.
btw, definitely do not use z-library or sci-hub to access these sources if they are paywalled.. thatd be just awful...
On Multiple Selves, By David Lester
This book is fantastic for talking about plurality not just in DID but in many different situations. It mentions the theories of dozens of psychologists which include the experience plurality-- not just in DID or pathology --showing that this subject IS well studied. A lot of these experiences and theories are remarkably similar to what endogenic systems commonly purport online as I will show in screenshots below. Even though there are screenshots i implore you to access this resource yourself, it's great and has so many different viewpoints on this subject.
Handbook of psychiatric measures (2nd edition., pages 587–599). by American Psychiatric Publishing, Inc.
Description: This resource is specifically on psychiatric measurement tools and diagnostic tools, but mentions DID and non pathological forms of plurality briefly. note "e.g." means EXAMPLE not the only possible experience of non-pathological dissociation.
Moreira-Almeida A, Neto FL, Cardeña E. Comparison of brazilian spiritist mediumship and dissociative identity disorder. J Nerv Ment Dis. 2008
Description: Studies about mediumship and a comparsion to those with DID, note that mediumship is being studied as a psychological dissociative phenomenon here and not particularly as a solely religious one, they are considering psychological explainations & factors. It is stated literally "there is empirical evidence that nonpathologic dissociative and mediumistic states have occured throughout history and in most societies". Though mediumship is often a spiritual experience I think the underlying psychological causes can be studied and equated to non-pathological/non-traumagenic plurality.
Lewis-Fernández, R. (1998). A cultural critique of the DSM-IV dissociative disorders section. Transcultural Psychiatry, 35(3), 387–400.
Description: Addresses concerns that the DSM-IV criteria for DID does not properly address non-pathological dissociation considered a part of normality in many cultures. Here it mentions that in some cultures dissociation in rituals and practices is normal and not pathological. I believe that this same view could also be applied to types of western endogenic plurality, as these also could be influenced by cultural factors as a form of dissociation that is not pathological. I would like to see more research specifically into western endogenic plurality!
If you want papers directly stating "Endogenic plurality is real & DID isnt the only way to experience it" there just probably isnt any. I dont think many of these researcher even know of online communities and our terms, but there is LOTS of research into plurality outside of DID & pathology, you just have to look for it and fit it to your use-case.
A lot of these are talking about spiritual and cultural experiences, this doesnt mean they arent real or arent based on underlying actual psychological experience, and most these studies/articles are specifically focusing on the psychological aspect. Mediumship is a common example of non-pathological dissociative experience, mainly because its more known and well studied, but I believe studies on western endogenic plurality would likely have similar studies. Nonetheless it does confirm that non-pathological & non-traumagenic dissociative experiences have led to plurality-like experiences, at least in some cultures around the world.