Eternalism and Mahayana Buddhism In this Wikipedia article on Arhat in Mahayana Buddhism, it is written:


  Instead of aspiring for arhatship, Mahayanins are urged to instead
  take up the path of the bodhisattva and to not fall back to the level
  of arhats and śrāvakas. Therefore, it is taught that an arhat must go
  on to become a bodhisattva eventually. If they fail to do so in the
  lifetime in which they reach the attainment, they will fall into a
  deep samādhi of emptiness, thence to be roused and taught the
  bodhisattva path, presumably when ready. According to the Lotus Sutra,
  any true arhat will eventually accept the Mahāyāna path.


In this Wikipedia article on the Eternal Buddha, it is written:


  In east-Asian Buddhism, the Buddha of the Lotus Sutra is regarded as
  the eternal Buddha. "The Tathagata´s Lifespan" chapter (ch 16) of the
  Lotus Sutra portrays the Buddha as indicating that he became awakened
  countless aeons ("kalpas") ago. The sutra itself, however, does not
  directly employ the phrase "eternal Buddha".
  
  In China the Lotus Sutra was associated with the Mahaparinirvana
  Sutra, which propagates the tathagatagarbha-doctrine, and with the
  Awakening of Faith. The Mahaparinirvana Sutra presents the Buddha as
  eternal, and equates him with the Dharmakaya.


If the Arahant in Mahayana Buddhism goes into a deep samadhi after death, till he wakes up and continues the path towards Buddhahood, and if after becoming a Buddha, his personality can persist as an Eternal Buddha, then this shows eternalism in Mahayana teachings.

An Arahant who can go into deep samadhi after death (without a body) only to reanimate later implies the existence of a soul.

Furthermore, the story of Buddha Prabhutaratna from Wikipedia also has elements of eternalism:


  "Great-Eloquence Bodhisattva" wants to see the Buddha in the stupa but
  Prabhūtaratna's vow makes it a prerequisite for showing his body that
  the Buddha who proclaims the Lotus teaching collects all his
  manifestations. At this point Shakyamuni summons from around the
  universe countless Buddhas who are his emanations, lifts the entire
  assembly into the air, and opens the stupa. Prabhūtaratna praises
  Shakyamuni and invites him to sit next to him. Shakyamuni then
  continues to preach the Dharma. In the 22nd "Entrustment" chapter of
  the Lotus Sutra, Prabhūtaratna and his stupa return to under the
  earth.


Eternalism (sāśvata-dṛṣṭi in Sanskrit) was rejected by the Buddha's original teachings on the Middle Path, as described by Ven. Dhammananda Mahathera's article (from here) excerpt below:


  Why did the Buddha deny the teaching of eternalism? Because when we
  understand the things of this world as they truly are, we cannot find
  anything which is permanent or which exists forever. Things change and
  continue to do so according to the changing conditions on which they
  depend. When we analyse things into their elements or into reality, we
  cannot find any abiding entity, any everlasting thing. This is why the
  eternalist view is considered wrong or false.


My question is:

Does Mahayana Buddhism teach eternalism? If this is not the case, then what is the right interpretation of the Mahayana teachings on Arahant and the Eternal Buddha above? 

In the question above, when I wrote "Mahayana Buddhism", I mean particularly in the context of the Lotus Sutra and the Mahayana Mahaparinirvana Sutra.
Several thoughts...

Mahayana is not a homomorphous teaching. It's a bunch of different schools, the only common theme of which is attempting to express The Truth in whatever crazy language that the recipient can understand. So you will see some Mahayana schools speaking about Eternal Buddha, or about Buddha Law, or about "True I", or a number of other topics that have some meaning for someone, and can turn into practice that will increasingly approximate "It", until they hopefully break through. All these different metaphors approach the truth in one way or another, they all have some sense in one way, and are false in another way. That's just the way with language unfortunately - any conceptual construct is always limited and onesided.

Another thought. Eternalism is a bad translation imho. I think a better term would be "transcendental substantionalism", as in, assuming that there is some magic substance or essence separate from the aggregates that lives on from one life to another.

If we assume, in modern terms, that material world, the world of energy transformation, and the world of information - are three ways to see the same world, and that as material forms arise and perish, energy and information transforms and fluctuates -- we can see that energy/information does not really "die" but it's not like it is "eternal" either. Then we can see how all these different metaphors were trying to point us to this truth, using whatever expressive means at their disposal, in the absence of better language we have today, like "information". Again, this is just yet another explanation, limited and onesided in its own way.

So when Lotus Sutra or Nirvana Sutra is speaking about Eternal Buddha - they just mean the Dharmakaya -- meaning, the information about Dharma latent in the world and spread around in little pieces, both as formal Dharma, as well as other sorts of hints and wisdoms. When this information accumulates in a single being in sufficient amount, and manifests itself as their way of living, we call that being a (Nirmanakaya) Buddha, and for the sake of explaining the nature of the phenomena we may employ the language of this being a reincarnation/manifestation of Eternal Buddha, and this can be useful to some people and confusing to others - but as long as we know what we are talking about, we will not be confused.

Can we call Dharmakaya eternal? Well in one sense, sure, it is not destroyed by individual deaths, and because it has its roots in the very nature of things, after all it's a teaching about how things work, it's inevitably "built-in" and is being discovered piecemeal by people in all times, even if formal Teaching is lost. In another sense, it's just a bunch of fragments, plus a potential to discover more fragments, so it's not even a "thing" to speak about, just a principle.