Eternalism and Mahayana Buddhism
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.