Hi Joel. My conclusions from my attempts at study are:
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The term ‘namarupa’ first appeared in what is called the Chandogya Upanishad. The Chandogya Upanishad is included in the Samaveda. Given the Buddha was aware of the Samaveda (as one of the Three Veda), it seems highly probable the Buddha was aware of at least the early parts of the Chandogya Upanishad or what became the Chandogya Upanishad. In the Chandogya Upanishad, the term ‘namarupa’ is not emphasized by the literal ‘name-form’. Instead, in the Chandogya Upanishad, namarupa refers to the Great Deity exercising their will to multiply the basic three deities (elements) of earth, fire & water into a multitude of names & forms. The primitiveness of the Chandogya Upanishad here, with its focus on ‘deities’, gives the impression to me (despite the different views of noted scholars) the Chandogya Upanishad is the oldest Upanishad.
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In the Suttas, the most common description/definition of namarupa found in SN 12.2, MN 9, etc, similar to the Chandogya Upanishad, also includes “the will” or “intention/volition” (“cetanā”). Therefore, my personal impression is SN 12.2 represents the original teaching of the Buddha and the use of namarupa is a redefinition of the deity based doctrine in the Chandogya Upanishad. In short, the definition found in SN 12.2 seems closer to “mentality” than to “name”. The “rupa” is definitely closer to “materiality” than to “visual form”, per the definitions of “rupa” found in suttas such as MN 62, SN 22.79 and AN 9.15.
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The other contender for the earliest Upanishad containing the term ‘namarupa’ is the Brihadaranyaka Upanishad. In the Brihadaranyaka Upanishad, namarupa has the generic & modern meaning of ‘name-form’ or naming the primal undifferentiated sense experience. Unlike the more primitive deity focused Vedic sounding Chandogya Upanishad, the Brihadaranyaka Upanishad emphasizes the modern Hindu notion of the Universal Atman that produces name-form, which seems a doctrine not discussed & debated in the suttas by Brahmins with the Buddha. Therefore, at least for me (but not for the mainstream establishment scholars), the Brihadaranyaka Upanishad was composed after the Buddha.
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Suttas such as DN 15, which is not mentioned in the later commentary called the Patisambhidamagga or mentioned in the Abhidhamma (which refer to SN 12.2 definitions), seem to include a definition of namarupa similar to the literal ‘name-form’ of the Brihadaranyaka Upanishad. For me, this, along with other evidence, points to DN 15 as being a later sutta, probably not spoken by the Buddha.
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While Bhikkhu Sujato probably does not agree with my conclusions about the two Upanishads above, Bhikkhu Sujato did write a very interesting introduction about the Digha Nikaya and how one of its purposes was for debating & converting Brahmins, here The Long Discourses: Dhamma as literature and compilation.
Therefore, in conclusion, my view is the translation of ‘mentality-materiality’ for SN 12.2, SN 12.67 & most of the SN best represents the teachings of the Buddha. Where as the translation of ‘name-form’ in suttas for Brahmins, such as DN 15, DN 11, MN 49, SN 7.6, etc, best represents these suttas. In other words, probably a single translation should not be adhered to. My impression is the current Western translators can be inflexible in their translations, such as always translating ‘dukkha’ in the same way (such as in SN 56.11 vs SN 22.59).
Also, those monks such as @Brahmali who strongly emphasize the Three-Life-Time interpretation of Dependent Origination I imagine favour the translation of ‘mentality-materiality’ because it seems obvious consciousness does not ‘descend’ or ‘rebirth’ into ‘name-form’. Where as those monks such as @ Kaṭukurunde Ñāṇananda and his unbending adherents who seem to verge on Solipsism tend to favour ‘name-form’. Interestingly, the scholar Buddhaghosa in his Visuddhimagga said ‘nama’ means ‘to bend’ or ‘to incline’, which may be supported by SN 12.38, SN 12.39 & SN 12.40 (in combination) and which also emphasizes the role of ‘the will’ (‘cetana’) & ‘attention’ (‘manasikara’) found in the definition in SN 12.2. Personally, I prefer ‘mentality-materiality’. 
Possibly also SN 12.38, SN 12.39 & SN 12.40 in combination.
Very good, Sir, very good. 