Thanks for the explaination. I can’t really say much else on the matter but do think that whether or not one is justified in concluding that it is a figure of speech warranting a non-litteral translation is from the looks of it a moot point. I also think that if indeed one was to claim that the expression is extraordinary, idiomatic, one should then provide extraordinary evidence.
As for amataparāyanaṃ amatapariyosānaṃ, imo, it does not constitute such evidence because these are not synonymous with culmination when taken as reaching “the highest point of development” or “climax”.
It’s when a mendicant develops right view, right thought, right speech, right action, right livelihood, right effort, right mindfulness, and right immersion, which culminate, finish, and end in the deathless.
Idha, bhikkhave, bhikkhu sammādiṭṭhiṃ bhāveti amatogadhaṃ amataparāyanaṃ amatapariyosānaṃ … pe … sammāsamādhiṃ bhāveti amatogadhaṃ amataparāyanaṃ amatapariyosānaṃ.
I think we can agree that Path leads to the Deathless and it is a Path to cessation of what is “Sabbe Dhamma Dukkha” or Dukkha in general.
Therefore it would be very strange if what is extinguished was said to gain a safe harbor in anything given that they are Ceased. If one was to explain Deathless as “a safe harbor” that would in itself arguably be a figure of speech akin to “Deathless being an unsurpassed security from bondage” and a similar expression does occur in the Dhp.
If however one goes with “grounded in” it puts an emphasis on the idea that what is extinguished is one thing and that in which it is grounded is another. In this sense it makes a lot more sense imho for if the grounded is extinguished it is natural that there is that which is the ground and the ground is unaffected by the extinguishment of that which gains a footing. Thus the ground is a single element whereas manyfold things can be gaining a footing ie many types of flowers can gain a footing in the same soil. In this sense there are two elements; one which varies (the grounded) and that which is unvaried (the ground) and this goes well with the two elements explained as Conditioned Element (various, manyfold) and the Unconditioned (unvarying singleness).
A way to think about it is considering the two elements Conditioned and the Unconditioned as a whole system. Some mendicants hold to the Idea that the Unconditioned has no Cosmological function and this is correct imo but even tho the Unconditioned is not a part of the Cosmology it is still an Integral part of the system as a whole, it is fundamental at that and if it wasn’t a part of the system then it would simply be a different system altogether and an imaginary one at that.
If one then agrees that unconditioned element is a fundamental part of the system as a whole it is then natural to infer that what is fundamental could be spoken of as the fundament, a foundation or the basis of something; a ground.