Is the Mahayana shunyata same as the Theravada papanca?

Hard to answer your question without categorising somehow. I will answer in pure sutric Mahayana terms, that is in terms of Yogacara which is a base from which later developments happen in Mahayana, it also exists in majority of Mahayana which is Chan/Zen and Pure Land.

The problem here is that Mahayana shunyata says even Nibbana is empty, but Theravada's sankhara does not include Nibbana. So, this does not match.

For instance within framework of both Sautrāntika and Yogacara, the general separation of purely conceptual phenomena would be non-existent and existent. Nibbana is existent conceptual phenomenon since it's possible (theoretically) to attain it versus impossible, like becoming a God, or a Papa Smurf.

Still, the difference among existent phenomena is that Nirvana is a concept that is relative to the various factors, it cannot be simply explained by mere ideas, it can only have a blueprint to which we, depending on situation and a given karmic burden, draw up and imagine different lines on the very sheet of paper. At a given moment it is like a single page in a colour book that we colour differently whenever it comes to mind. It is not a general characteristic of anything graspable, it cannot come from the touch with reality, it is not there and it does not function. This is not what is claimed in Theravada, things are Good or Bad and Samsara is opposed to Nirvana as hard, concrete things; in here (non-sunyata) there is little room for liberation based on reflection on absolute dualities formed by some inherent bias or absolutist tendencies.

So, does it make sense to say that the Mahayana shunyata is same as the Theravada papanca?

So, in the light of this question, let us navigate towards dependent things - "real" things, like a flower. Whatever is there in reality, in terms of Theravada, is the direct source of representation of the object. That means that object of reality already has all the labels and characteristics (i.e. colour, size, shape etc), and conceptually they arise from the vision of the earth for instance. It is as if to say that these objects already possess these characteristics that make it earth. That is very different from later Mahayana developments.

Firstly, there are two layers added on top of that in Mahayana related to phenomena. On the conceptual level, as I have mentioned, in the whole Mahayana vs Theravada debate, there is inherent divergence regarding things conceptual like Samsara and Nirvana. This layer of purely conceptual things not being real was already added by Sautrāntika, and was a first distinction between early Theravadin understanding. But on dependent phenomena level, "real" level it is pretty much the same. By the same I mean that in Sautrāntika when one is looking at things non-conceptually, this is what happens: all nonstatic phenomena that perform functions truly exist and give characteristics to conceptual phenomena. At this point Sautrāntika had single point of touch with traditional Theravada, but Yogacara diverged in this point completely, and this complete diversion constitutes Mahayana's sunyata.

Yogacara proposed that at this point, everything that we perceive non-conceptually is based on a first, bare perception of consciousness (by alayavijnana or senses), and that is instantly tainted by karmic seeds - those are preconceptions we developed in the past, like being prejudiced against Islam. We actually perceive it already tainted, even non-conceptually. This implies complete nonduality of the natal source of where objects and mental phenomena come from and manifest to us. Here, things appear from mind both conceptually and non-conceptually (mind-only). They don't come characterised from external source but Self pre-characterises them from the start. Immaturity of Self grasping aspect (Manas) and past karma characterise them. All of these preconditions are capable of creating mood and feeling or sensation before a thought or actual deliberate grasping is to be perceived and neutralised. This is seen in the way of not being able to release oneself from fixations, bad habits or having prejudices by attaining some views. The key is ultimately in karma and transforming karmic seeds to transform how objects appear to us in the first place.

From that point a base was formed for further elaboration by Tibetans for example, they especially developed these notions in non-sutra and strictly speaking not "core" Mahayana.

Ultimately, from my point of view, and note that this is just my humble opinion, the concept of sunyata is important for understanding the path to liberation.

In the context of what I have written, Nirvana is empty, it is not a real thing, it comes from the mind and mind is the monarch. It makes sunyata very very different from papanca. In the way that it is much much more elaborated, philosophically.

Sunyata makes sense within the context framework of reconciling two truths; conventional and ultimate, how do they relate to each other, whereas papanca is simply about Self apprehension of the objects that exist as true external sources of characteristics, which is negated in the former. In sunyata, no absolutely infallible reality exists as a direct source to mental fabrications due to posing additional karmic layers in-between.

Important disclaimer: By saying that Mahayana diverges or disagrees with Theravada, I don't meant that things were that crystal clear in Theravadin context; much of what is today implied is just a result of interpretation of something that wasn't deliberately confirmed, nor denied by Buddha.

Another point is that representations of sunyata along with commentary are vast and have some differences (especially in Tibetan context). To actually try to say that it is like this or like that in this context here, would not make me Mahayanist at all, but a dualistic old fart. Thus, I will not make such fixed attempts and chose Yogacara as a reference point, since it has common grounding foundation in most schools.

The problem here is that Mahayana shunyata says even Nibbana is empty, but Theravada's sankhara does not include Nibbana. So, this does not match.

Theravāda's suññata, anattā, included nibbāna. But mahāyāna's nibbāna is attā of Theravāda. It is the same reason of nikāya-separation in Abh. Katha. age as well.

VN Parivāra:

Saṅkhāra, which means saṅkhata(effect-saṅkhāra)*, is anicca&dukkha&anattā;

But only nibbāna&paññatti which are explained just as anattā.

KN Dhammapada has the same explanation.

Prapanca and Sunyata are two different things altogether, in fact, they both have somewhat opposite meanings, the Sunyata can be experienced after attaining Nibhana, but Prapanca is empty words or worthless talk that can create causes (karma) hence can never attain Nibbhana. To create a cause the primary source is sankhara and it has three types of emotional experiences, those are - like, dislike and equanimity, basically Upaadana (clinging) sanskaras are- likes and dislike ( attraction and repulsion), these two types of mental formations which are the primary source of Re-Birth process hence Prapanca (empty or baseless or pointless talk) is due to ignorance.