Thank you Prof. Torella for your clarification. One point of disagreement here: you say "in these contexts saṃskāraḥ always refers to the residual traces left by “inferior” realities on the way of their being progressively overcome", but we have a clear example to the contrary in Kṣemarāja's P.H. sūtra 19 and comm, where samādhi-saṃskāra clearly refers to a beneficial and desirable saṃskāra. But that aside, I propose a different reading of tatsaṃskāraḥ in the passage below, though not with full confidence. From everything we have learned in this alchemical discussion (and following up the references given), drutarasaḥ in the metaphor can only be the mercury, so here drutarasaḥ must analogically stand in for cidrasaḥ (or prakāśarasaḥ), hence the iva. As seen in real-life amalgamation, the drutarasaḥ indeed ābhāti kevalam -- appears alone -- as a result of this process, having digested the mercury. So Abh. is saying that cidrasaḥ is what remains, having devoured prāṇa-dehādi-dhātuḥ (cf. jīrṇa here with grasa, alaṅgrasa elsewhere in the literature). That leaves us with the difficulty involving tatsaṃskāraḥ. Since Abh. did not write tatsaṃskārasahitaḥ but easily could have done, I propose that he might be using saṃskāra here in its other primary sense -- that is, the drutarasaḥ is the "purifier of that (prāṇa-dehādi-dhātuḥ)" since that is exactly what mercury does. But I withdraw my guess that tatsaṃskāraḥ meant "the impression of awakened consciousness itself" which Dr Torella rightly called unconvincing.
But I think it cannot be true that tatsaṃskāraḥ here means "the saṃskāras left by body, prāṇa, etc. [for the moment still remain]" as Prof. Torella suggested, because the turyātīta here being discussed is being contrasted to a previous turyātīta (there being two versions of this attainment), about which it was said: śūnyādi-saṃskāro 'pi asti, which is unambiguous. How can there be a contrast if the saṃskāras remain in this second turyātīta as well? (Unless I'm missing some other point of contrast.) I take the point that if one becomes ekībhūtam sa.mvid-ghanam then there is no samāveśa possible, but I'm not clear that Abhinava disallows this possibility of going beyond samāveśa.