What is panna? Is it permanent?

The panna of an arahant is permanent during the arahants lifetime, per MN 12, which says:

Sariputta, even if you have to carry me about on a bed, still there will be no change in the lucidity of the Tathagata's wisdom.

But because the mind of an arahant ceases when life ends, obviously, panna does not last forever. Therefore, ultimately, panna is impermanent.

Nibbana is an unconditioned independent element of nature that permanently exists forever. The suttas says regardless of how many Arahants abide in the Nibbana element, the amount of the Nibbana element never ceases, changes or fluctuates (AN 8.19).

But panna is a conditioned mental element that is impermanent. For example, when there is no arising of Buddhas, there is no real panna in the world. There are times when panna does not exist; such as when Dhammadhatu is suspended from BSE.

No, it's not nicca, but the final means for liberation. Pañña-vimutti. This path is for liberation, and for nothing else. Of course "knowledge about liberation" will be ones last and most helpful knowing, wisdom. Liberated, unbound, there is no more use of this, saṅkhāra of this raft.

May one investigate the heartwood-similie suttas on this account.

[Note that this isn't given for stacks, exchange, other world-binding traded, but for liberation only]

sutta explain that panna [discernment] and consciousness are conjoined.

What one cognizes, that one discerns. Therefore these qualities are conjoined, not disjoined, and it is not possible, having separated them one from another, to delineate the difference between them.

consciousness is to be understood and panna is to be developed.

developing panna one comes to understand consciousness, having understood consciousness one has developed panna.

Suppose a person attains full understanding of consciousness but lacks in the power of concentration. Suppose this person then dies without having attained Arahantship and is born again.

It is not same body that is born and it is not the same faculties that are consequently constructed but the body and faculties that come to be are a result of previous development.

There is no discernable point in the past where there was no delusion with a consequently discernable appearance of delusion.

Perfected Panna has such result that it always leads to the consequent perfection of the other faculties in no more than 7 births. The perfection of faculties results in extinguishment of conditions that lead to death & birth altogether.

Therefore if faculties had ever been brought to culmination in the past, then this birth wouldn't have occurred.

Light is movement of photons which vibrate within a certain frequency range and travel at the speed of light. If there there is no one present with eyes to sense light, is there still light? I would say yes.

Nibbana is a permanent phenomena (dhamma) which is like light. Even if there are no enlightened ones to experience it, it is still there.

From MN 49 with corrected translation from this answer:

That which could be known (Nibbana), is invisible, infinite, radiant all round—that’s what is not within the scope of experience based on earth, water, fire, air, creatures, gods, the Creator, Brahmā, the gods of streaming radiance, the gods replete with glory, the gods of abundant fruit, the Overlord, and the all.

Also Ud 8.1-4.

But wisdom (panna) is different. It depends on the mind. If the mind ceases, then wisdom ceases. When the Buddha passed away, his wisdom ceased, but his teachings (Dhamma) which arose out of his wisdom, continue to exist through the suttas and the noble sangha.

So are the teachings (Dhamma) permanent or unconditioned? I would say no. If the suttas and the noble sangha disappear after many centuries or millennia (i.e. the end of the current Buddha sasana), then the teachings also disappear.

Some have asked if physical space is permanent or unconditioned. From Einstein's theory of general relativity, we know that physical space-time is conditioned.

So, in every case, we can see that only one phenomena is permanent and unconditioned and that is Nibbana. It is that which is experienced by the mind when it is free of defilements and fetters. It is not a mental idea or state of mind or state of consciousness.