“What happens” does not apply because there is no “after” or “future” for this of that being who attains Parinibbana;
"Now what, bhikkhus, is the Nibbana-element with no residue left? Here a bhikkhu is an arahant, one whose taints are destroyed, the holy life fulfilled, who has done what had to be done, laid down the burden, attained the goal, destroyed the fetters of being, completely released through final knowledge. For him, here in this very life, all this is experience, not being delighted in, will be extinguished. That, bhikkhus, is called the Nibbana-element with no residue left.
“These bhikkhus, are the two Nibbana-elements.”
Verse:
These two Nibbana-elements were made known
By the Seeing One, stable, and unattached:
One is the element seen here and now
With residue, but with the cord of being destroyed;
The other, having no residue for the future,
Is that wherein all modes of being utterly cease. (Itivuttaka 2.17)
You are trying to talk about what happens to what is conditioned (an arahant) after cessation of conditioned phenomena. Which is not something that ought to be done and you are going too far.
There is no going beyond Parinibbana, there is no “coming out of conditioned phenomena” or “conditioned phenomena going to some place” therefore Sutta are very explicit language such as;
The other, having no residue for the future,
Is that wherein all modes of being utterly cease.
Talking about after Parinibbana does not apply, again, here Maha Kotthita asks similar questions;
[Maha Kotthita:] “Being asked if, with the remainderless stopping & fading of the six contact-media, there is anything else, you say, ‘Don’t say that, my friend.’ Being asked if … there is not anything else … there both is & is not anything else … there neither is nor is not anything else, you say, ‘Don’t say that, my friend.’ Now, how is the meaning of your words to be understood?”
[Sariputta:] "The statement, ‘With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?’ objectifies non-objectification.[[1]]
The statement, ‘… is it the case that there is not anything else … is it the case that there both is & is not anything else … is it the case that there neither is nor is not anything else?’ objectifies non-objectification. However far the six contact-media go, that is how far objectification goes. However far objectification goes, that is how far the six contact media go. With the remainderless fading & stopping of the six contact-media, there comes to be the stopping, the allaying of objectification. Kotthita Sutta: To Kotthita
Here is another one which goes deeper but also does not go on to objectify non-objectification;
“What do you think, Anuradha: Is form constant or inconstant?”
“Inconstant, lord.”
“And is that which is inconstant easeful or stressful?”
“Stressful, lord.”
“And is it proper to regard what is inconstant, stressful, subject to change as: ‘This is mine. This is my self. This is what I am’?”
“No, lord.”
“Is feeling constant or inconstant?”
“Inconstant, lord.”…
“Is perception constant or inconstant?”
“Inconstant, lord.”…
“Are fabrications constant or inconstant?”
“Inconstant, lord.”…
"Is consciousness constant or inconstant?
“Inconstant, lord.”
“And is that which is inconstant easeful or stressful?”
“Stressful, lord.”
“And is it proper to regard what is inconstant, stressful, subject to change as: ‘This is mine. This is my self. This is what I am’?”
“No, lord.”
“What do you think, Anuradha: Do you regard form as the Tathagata?”
“No, lord.”
“Do you regard feeling as the Tathagata?”
“No, lord.”
“Do you regard perception as the Tathagata?”
“No, lord.”
“Do you regard fabrications as the Tathagata?”
“No, lord.”
“Do you regard consciousness as the Tathagata?”
“No, lord.”
“What do you think, Anuradha: Do you regard the Tathagata as being in form?.. Elsewhere than form?.. In feeling?.. Elsewhere than feeling?.. In perception?.. Elsewhere than perception?.. In fabrications?.. Elsewhere than fabrications?.. In consciousness?.. Elsewhere than consciousness?”
“No, lord.”
“What do you think: Do you regard the Tathagata as form-feeling-perception-fabrications-consciousness?”
“No, lord.”
“Do you regard the Tathagata as that which is without form, without feeling, without perception, without fabrications, without consciousness?”
“No, lord.”
“And so, Anuradha — when you can’t pin down the Tathagata as a truth or reality even in the present life — is it proper for you to declare, ‘Friends, the Tathagata — the supreme man, the superlative man, attainer of the superlative attainment — being described, is described otherwise than with these four positions: The Tathagata exists after death, does not exist after death, both does & does not exist after death, neither exists nor does not exist after death’?”
“No, lord.”
"Very good, Anuradha. Very good. Both formerly & now, it is only stress that I describe, and the cessation of stress."Anuradha Sutta: To Anuradha
If we look at the Sutta it seems to me that whenever the Unmade is talked about in a sense of some sort of reality of it’s own it is always talked about in a paradoxical terms;
However when questioned further there are these explainations;
‘Gotama the contemplative speaks of the cessation of perception & feeling and yet describes it as pleasure. What is this? How can this be?’ When they say that, they are to be told, ‘It’s not the case, friends, that the Blessed One describes only pleasant feeling as included under pleasure. Wherever pleasure is found, in whatever terms, the Blessed One describes it as pleasure.’" Bahuvedaniya Sutta: Many Things to be Experienced
Or Sariputta;
“Then there is the case where a monk, with the complete transcending of the dimension of neither perception nor non-perception, enters & remains in the cessation of perception & feeling. And as he sees (that) with discernment, effluents are completely ended. So by this line of reasoning it may be known how unbinding is pleasant.”
There just isn’t much explaining going on as we can see.
As for discernment this is also one of the way it is spoken of in form of affirmation;
There is, monks, an unborn[[1]] — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that escape from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, escape from the born — become — made — fabricated is discerned.[[2]]
Nibbāna Sutta: Unbinding (3)
By inference it’s itness can be established but not defined here.
Afaik the last expression of positive affirmation of the Unmade is Ud 8.1;
There is that dimension, monks, where there is neither earth, nor water, nor fire, nor wind; neither dimension of the infinitude of space, nor dimension of the infinitude of consciousness, nor dimension of nothingness, nor dimension of neither perception nor non-perception; neither this world, nor the next world, nor sun, nor moon. And there, I say, there is neither coming, nor going, nor staying; neither passing away nor arising: unestablished, unevolving, without support. This, just this, is the end of stress.
When taken as riddle it really narrows it down tho but also here it is explained in terms which it is not as it’s itness is affirmed so it does not really go to objectify it.
As for unestablished consciousness it may well be that my expression when saying;
“Unestablished consciouness ceases”
is not the Classical Theravadin Abhidhamma Method expression but the meaning is that it ceases in as far as the Aggregates which can be grasped with wrong view to be personal or belonging to this or that person, all that, is utterly extinguished.
As for the term unestablished in ud8.1 i think one can draw a link between the unestablished here and unestablished consciousness in case of an Arahant however i think that then one would be going by “Abhidhamma Method” and this is an important distinction, by the Abhidhamma method one can go on to explain Mind-Base as being conditioned or unconditioned and within that framework i do see it work very well but that expression is definitely much more comprehensive & intricate and therefore requires context to work because it ties together the doctrine in it’s entirety and requires Abhidhamma understanding and otherwise it would be one shade away from eternalism and a soul imho.