Is rebirth a conventional truth within the Buddha's message?

Rebirth is an ultimate truth as when on set of aggregates disintegrate a new set if formed while physical death and rebirth is a special case of this process.

Each being is a dependently arisen process. This process is also not be taken as self, but as a natural cause karmic effects arise as well as rebirth.

Also the rebirth process involves Citta and optionally Nupa which are part of the ultimate realities.

Put it into simplicity, if you died, if there's no rebirth, where have you gone? Will you one day "die"? Of course. Thus the dependent notion (相待) "rebirth" can't be cancelled. It's just that simple, it's a fact.

In dependent origination, 'jati' ('birth') refers to the conception & production of the view of 'beings' ('satta') based in the beguiling appearance & manifestations of various aggregates and the mind seized by & enslaved to sense objects (ayatana).

Comprehending 're-birth' is therefore necessary to understand the Buddha's message & to reach enlightenment.

The Buddha did not teach the two truths doctrine. The defiled/polluted mundane right view in MN 117 is not really 'the truth' because it is defiled with upadhi (attachment;self-views). It is only a right view that accords with morality (rather than accords with Nibbana).

'The two truths' were developed a little later on by Buddhaghosa. However, again the two truths of Buddhaghosa are not really both truth because one truth is how the truth really is and the other truth is the worldly interpretation by the unenlightened. Buddhaghosa said:

The Awakened One, best of speakers, Spoke two kinds of truths: The conventional and the ultimate. A third truth does not obtain.

Therein: The speech wherewith the world converses is true On account of its being agreed upon by the world. The speech which describes what is ultimate is also true, Through characterizing dhammas as they really are.

Therefore, being skilled in common usage, False speech does not arise in the Teacher, Who is Lord of the World, When he speaks according to conventions.

(Mn. i. 95)

In the language of ultimate truth, since 'rebirth' simply means the re-arising of 'self-view', it is an ultimate truth because this is how suffering always occurs.

'Re-birth' or 'new-birth' is conditioned by new or re-becoming, as explained in dependent origination.

This re-birth will inevitably result in 'death' ('marana'); that is, the painful sense the born 'self' or 'self-identity' has lost something; or otherwise the sense that other 'beings' ('satta') have also lost something.

When the lesson of marana & resultant dukkha is not learned, ego birth will occur again in the mind, which is re-rebirth.

Rebirth is a reality that can be affirmed when considering ultimate truth because it happens many times each day for the non-enlightened.

Each time the mind believes & acts with craving in terms of an imagined "I", "me", "mine", "you" etc, that is 're-birth'; the mental production of the view of 'beings' ('satta'). '

Ajahn Chah said:

''Becoming'' (bhava) means ''the sphere of birth.'' Sensual desire is BORN at sights, sounds, tastes, smells, feelings and thoughts, IDENTIFYING with these things. The mind holds fast and is stuck to sensuality....Bhava is the preliminary condition for birth. Wherever birth takes place, that's bhava. For example, suppose we had an orchard of apple trees that we were particularly fond of. That's a bhava for us if we don't reflect with wisdom. How so? Suppose our orchard contained a hundred or a thousand apple trees... it doesn't really matter what kind of trees they are, just so long as we consider them to be ''our own'' trees... then we are going to be ''born'' as a ''worm'' in every single one of those trees. We bore into every one, even though our human body is still back there in the house, we send out ''tentacles'' into every one of those trees. Now, how do we know that it's a bhava? It's a bhava (sphere of existence) because of our clinging to the idea that those trees are our own, that that orchard is our own. If someone were to take an ax and cut one of the trees down, the owner over there in the house ''dies'' along with the tree. He gets furious, and has to go and set things right, to fight and maybe even kill over it. That quarreling is the ''birth.'' The ''sphere of birth'' is the orchard of trees that we cling to as our own. We are ''born'' right at the point where we consider them to be our own, born from that bhava. Even if we had a thousand apple trees, if someone were to cut down just one it'd be like cutting the owner down. Whatever we cling to we are born right there, we exist right there. We are born as soon as we ''know.'' This is knowing through not-knowing: we know that someone has cut down one of our trees. But we don't know that those trees are not really ours. This is called ''knowing through not-knowing.'' We are bound to be born into that bhava. http://www.ajahnchah.org/book/Flood_Sensuality1.php

Rebirth is very central to the whole doctrine. As long as this Kammic force exists there is re-birth, for beings are merely the visible manifestation of this invisible Kammic force. Any attempt is like excluding Samsara is like excluding this from the whole equation. Birth precedes death, and death, on the other hand, precedes birth. The constant succession of birth and death in connection with each individual life flux constitutes what is technically known as Samsara -- recurrent wandering. Buddha said that:

"Without cognizable end is this Samsara. A first beginning of beings, who, obstructed by ignorance and fettered by craving, wander and fare on, is not to be perceived."

The cause of this Kamma, continues the Buddha, is avijja or ignorance of the Four Noble Truths. Ignorance is, therefore, the cause of birth and death; and its transmutation into knowingness or vijja is consequently their cessation. The result of this analytical method is summed up in the Paticca Samuppada. Paticca Samuppada is the discourse on the process of birth and death.

Nibbana is often conceived of as stopping this cycle. By removing the causes for craving, craving ceases. So with the ceasing of birth, death ceases. With the ceasing of becoming, birth ceases... and so on until with the ceasing of ignorance no karma is produced, and the whole process of death and rebirth ceases.