I am one of those who mentioned denial coming into play. So I guess I need to clarify something here. I do not attribute the subjective experience of health to denial. Its the other way around. What I have observed in myself is that when I have a subjective experience of singularity, I begin to doubt that I actually have DID or that I need any help. I minimize and dismiss and invalidate. I imagine I could just sweep all the DID phenomena including the alters under a mental rug all over again, as I have in the past, and get on with my life. But why should they stay hidden under the rug this time?
I didn't have you in mind so much with that as some discussions I remember from a while ago, e.g. one where a person got flamed off the board because they suggested that a member who experienced a temporary change in their perception may have improved rather than gone into denial (ok, the person was being obnoxious as well). Of course I cannot know whether anyone experiences improvement (if you want to call a move toward integration that) or denial. But neither can anyone else from the outside.
Just to play the devil's advocate here for a minute: how do you really tell the difference when someone has a subjective experience of "aloneness"? The person may have dissociated further, but they may also have moved closer to integration. But on here (that's my subjective perception) there seems to be a certain bias toward the former, i.e. people are more likely to attribute this feeling to denial than improvement. I'm not saying it cannot be that, just that we don't necessarily know. So e.g. when someone gets the doubts that you mentioned, it may well be that they're in denial, but it may also be that they really do need less help or that the DID is far enough in remission to not trouble them at that time. The only person who can tell for sure is themselves.
In my case this is not an assumption; this is what I experience. They are still in there, and they have continuity. I met Alter 1 and Alter 2 decades ago, then suppressed them (or so I thought), and in crisis there they were again, the same two entities. And in the meantime they had accumulated decades of life history and experience. They had been awake and using passive influence and communicating to me in rather covert ways.
I am in no position to comment on the contents of anyone's head but my own, but just for the sake of argument: if it is true that the self is a model that the brain constantly creates and re-creates and the same goes for multiple selves, then could it not be possible that something happens like:
a person walks around for decades with what they believe is a unified, singular self. unbeknownst to them, they are repressing certain memories associated with trauma. Now they come into a situation where the traumatic memories are triggered (e.g. because something reminds them of the original trauma) and come to the surface.
The brain in this scenario cannot reconcile the information based on the persons "normal" consciousness with the information based on the trauma memory. so it reacts by creating two disparate self-models based on either. the point is, in this scenario, the alter consciousness would be created at the time of triggering rather than being there in latent form.
of course that begs the question why alters would come with continuous memory and a personal history. I think it is possible that they draw on a latent memory bank that was previously inaccessible but not conscious. when the traumatic content is triggered, the subsequently created self-model has access to this history and thus experiences itself as a continuous narrative entity, even though in their dormant state they did not actually "exist" in that sense.
This idea is much less "far out there" considering that some researchers like Thomas Metzinger believe that the same goes for singular selves - when we are asleep or unconscious, the self does not actually exist but is re-activated upon waking and feels like it has continuous existence even though it technically doesn't.
Btw, I don't mean to infringe on anyone's experience with this stuff, I'm just genuinely interested.
It shows an excessive tenderness for the world to remove contradiction from it and then to transfer the contradiction to reason, where it is allowed to remain unresolved.
G.F.W Hegel