How do classical Mahāyāna, Yogācāra, and later nondual schools (e.g., Zen) articulate nonduality without reintroducing metaphysical eternalism?

In contemporary scholarship on Buddhist philosophy, nonduality is an important theme, but its ontological and epistemological status varies greatly across traditions. For example: Madhyamaka critiques any intrinsic nature (svabhāva) and affirms nonduality as a de-reification of both subject and object. Yogācāra is often interpreted as asserting mind-only (cittamātra), but classical Yogācāra philosophers also defend a two-truths framework to avoid ontological commitments. Zen emphasizes direct nondual experience, yet it operates outside detailed philosophical articulation. Question: How do these various Buddhist approaches such as classical Mahāyāna/Madhyamaka, Yogācāra, Zen, etc conceptualize nonduality such that: It does not collapse into a metaphysical foundation or eternal ground (i.e., avoids eternalism / ground-substantiation), It preserves Buddhist soteriology (dependence, emptiness, two truths), And it remains philosophically coherent within each school’s own ontological and epistemic frameworks? In other words: What are the distinct mechanisms or philosophical moves each tradition uses to articulate nonduality without turning it into a reified ultimate reality? Please support answers with primary sources or credible secondary scholarship where possible.