Q. Nagarjuna's Mūlamadhyamakakārikā contains various refutations of causation, change, "self-existence"/essence/svabh̄ava, and that perceptions and conceptions thereof are illusory and there is only "thusness"/tathata. For example, his tetralemma about causation comes to the conclusion that a thing could neither arise from something identical nor from something different, nor from both or neither (I will ignore the last one as I'm not sure anyone actually believes something can be derived from nothing). Therefore causation cannot actually take place. This is not only an attack on causation but also essence. The Platonist position in this framework, I suppose, would be closest to 3): the effect of a cause is both identical and different from that which is caused yet still possesses a certain self-existence/essence (we do not claim that it possesses independent existence other than the One who is beyond existence, nor do I think any spiritual system does). To Nagarjuna this position is absurd - how could an essence that belongs wholly to itself and is by nature eternal be caused and cause something posterior to it in turn, and, in the case of the eternal soul, undergo any sort of change? How do we square away the ascent of the soul from bodily existence to the Absolute if it is by nature unchangeable, especially since the intermediate - nous, the Divine Mind - is itself atemporal and the soul would have to reside there before finally rejoining the One? On a related note, if the soul once fully identified with the One then how could re-joining the One be permanent? This implies the state it had before its "fall" from the One was in some ways imperfect but the state it acquired after rejoining the One permanently is more perfect or absolutely hyper-perfect. All of these issues can be extrapolated I think from Nagarjuna's MMK, and that, given all these contradictions, it seems erroneous to think in terms of essence, change and causation as Nagarjuna claims.
Now, let me state that I don't believe Mahayana Buddhism actually escapes these conundrums. There's a not-so-subtle sophistry in their claim that while conventional reality is illusory we can still use it to arrive at "thusness", as this implies that the illusion somehow participates in "ultimate reality", meaning instead of being totally and fully illusory there are degrees of reality - this is no different from what we call ontology, in fact it mirrors the Plotinian model of reality. The conventional reality we perceive, instead of being totally illusory, is therefore but a mirror image of actual reality. Of course, at the level of the Absolute there is no causation, change, movement, difference, etc. However, if we follow Nagarjuna's logic to its actual conclusion (instead of the sophistic escape of the two truths doctrine) there would actually be no way for anything to arrive anywhere meaningful. In short, Nagarjuna's conclusions are as absurd, if not moreso, then that which he refutes.
All this infers that Nagarjuna's conclusions must in some important ways be erroneous and that, at the very least, Mahayana Buddhists do not truly believe what the MMK states and escape from their own philosophical foundations by means of sophistry and rhetorical sleights of hand. However, it is just that: inference and not a direct confrontation with Nagarjuna's claims. The actual truth behind the MMK, contra to what Nagarjuna himself claims, would be a sort of staticness and an "absolute un-truth" that we could never hope to escape. The only other option would be a sort of ontology and it is telling Mahayana Buddhism sneakily opted for the latter. However, I find myself unsatisfied by this indirect refutation and at unease. I believe there must be some way to directly confront Nagarjuna's reasoning instead of merely pointing out the absurdity of its consequences.
(Original question abbreviated and edited.)
A. I wouldn't worry about all this stuff. These concerns are not necessary and are not based on valid assumptions. Nagarjuna's/the MMK's analysis or 'logic', along with the doubts that it makes you give rise to, is based on invalid, realist/nominalist, non-idealist, and unexamined premises that limit reality and the possibilities and explanations just to the lowest phenomenal level, which has minimal reality. These questions and supposed contradictions about causation, change, etc. could only arise if the attempt to explain causation, etc. is limited to just the lowest plane of phenomenal and supposedly material things. Causation and phenomenal spatial and temporal change are readily understood without contradiction as the projection of the order and structure of nous onto or into the lower level of non-being, as the life of soul in time and space unwilling to take the whole of nous all at once. There is only, as Plotinus puts it in Ennead 3.8, contemplation--stronger and weaker, fuller and less full, complete and incomplete, clearer and more obscure, but still only contemplation. The One/Good is the completely full clear contemplation; the life of the most degraded souls is the weakest most incomplete and dark contemplation. The ascent, and also the descent, of the soul is just a change in the state of contemplation, so there is no problem of an eternal essence undergoing some sort of change analogous to a sort of physical change. Also, it is not like the soul was a physical part of the One before its fall and then physically rejoins the One, as the language of your doubt seems to tacitly suppose. The potential for not knowing is present within knowing, the potential for darkness is present within light, for non-being within being. When the soul completes contemplation and fully knows and becomes the light, the not knowing and darkness and non-being is gone. You don't have to worry about it recurring. All this is actually kind of ineffable and only really known in contemplative ascetic experience, since all this discussion and questions and process only occurs at the lowest least real level.
Nagarjuna/the MMK is maybe just being what the Platonic dialogs call an "antilogikos" or a "sophist taking refuge in the darkness of non-being". Perhaps, you might want to review the discussion about not becoming "misologikos" that refers to "antilogikos" people that are pleased within themselves when they turn everything up and down to make it seem that no definite conclusions are possible in the Phaedo at 89b-90d. (Actually, it wouldn't be a bad idea to read the whole Phaedo carefully say 50 or 100 times!). Other dialogs also deal with this problem and the kind of difficulties that the antilogikoi and sophists of the world cause. You also may want to eventually read through all of the Enneads, even though this is a big project and you won't likely understand much of what you read for a long time, as Plotinus deals with the kinds of concerns you have. You don't have to be able to understand everything at the discursive level before proceeding with the ascesis. Indeed, it is impossible to do so. And being and working as a celibate, vegan, teetotalling, etc. ascetic practicing contemplation is as necessary for coming to even a discursive level understanding of these things as racking your brains over Nagarjuna's attempts at logical contortion.
I don't know if it will be of any help or interest, but here is a link with an added introduction of the original sort of non-sectarian version of one of my pieces that perhaps you have seen in the specific Platonist form that I was asked several years ago to write for an old English language Indian magazine: https://www.csp.indica.in/an-indo-hellenistic-understanding-of-contemplative-asceticism-eric-s-fallick/
Perhaps, it will be some consolation to you to see that there is a great deal one can do to proceed on the Path and that the essentials of doing so are clear even before necessarily resolving all the possible conceptual doubts that may arise on reading the different texts of the different systems.
