Is nothing but all-encompassing, absolute Nibbana without Samsara possible? If so, is sustainability possible (and how)?
You assume a fundamental division but we are told (for instance by Rumi) 'the two worlds are one'. My guess or conceptualisation would be that Samsara and Nirvana are co-dependent and reduce to Unity. This would suggest that Samsara and Nirvana are dual-aspects thus inseperable.
Just pondering out loud. This is beyond my pay-grade. I'm not even sure this language is appropriate.
EDIT: No wait. This is not beyond my pay grade. Samsara and Nirvana are said to have a dependent-existence and to not really exist. Nagarjuna proves it. The truth would be beyond intellection or conceptualisation and to entail the dissolution of all conceptual opposites including Samsara and Nirvana.
So the short answer would be 'no'.
It's not so much: is it possible to have Nibbana without Samsara? That is just how it is. Positive is accompanied by negative, yin and yang, pain/trouble and peace/bliss. The goal is to be more towards Nibbana, and step out of the way of, or avoid being in Samsara. Beings learn by falling into Samsara, and deciding not to go back there!
I think that nibbana is defined as the end of suffering -- which occurs with the end of craving (as defined by the noble truths).
The end of craving happens (permanently) when the various causes of craving up uprooted -- the causes of craving include e.g. anusaya and asava and so on.
This uprooting can be permanent in the same way that when a palm tree is cut then it won't grow again.
Conversely, Samsara is a cycle of existence -- the pratītyasamutpāda doctrine (i.e. the doctrine of the 12 nidanas) says that cycle (including further birth) is conditioned by craving -- and so does this description:
“Mendicants, transmigration has no known beginning. No first point is found of sentient beings roaming and transmigrating, hindered by ignorance and fettered by craving.
Buddhism doesn't define the extremes; Samsara's beginning and end is not apparent. So, if it's not apparent you question is, according to Buddhism, wrong question.
Sure, that's what the path of the Noble One is all about. While the question "is sustainability possible (and how)?" is of no need. It's the very nature of Unbond that it does not require any sustainability, entertainment, is free of "under holding", holding on, requirement of support. For one who as fulfilled the task.
[Note: this is a gift of Dhamma, not thought for trade, stakes, exchanges or other gains subject toward decay and should be deleted if it's not giften to give in Dhammic conditions]