The Problems Posed by Isaiah to Trinitarianism in light of Psalm 110:1

I have discussed Isa 43:10 and other like-texts that Trinitarians have used against Latter-day Saint theology, including the following:

Attempted exchange with Desmond Ferguson (formerly of Irish Church Missions)

Is Isaiah 44:24 in conflict with Latter-day Saint Theology?

The rhetoric of Isaiah and the supremacy of Yahweh

I recently posed this challenge to a Trinitarian critic of the LDS Church about these texts where Isaiah is speaking about Yahweh, which shows the theological and exegetical bind Trinitarians place themselves in if they appeal to such passages:

I am asking you which divine person is speaking. It is YHWH--in your view, is this the Father? Son? Spirit? If all three, why the singular personal pronouns and singular verbs? (unless you are a Modalist who views all three as a singular person .[I know you are not]).

 If you claim YHWH and the three persons of the Trinity are one and the same (some go down this route with these passages), then what about texts where Jesus is distinct from Yahweh? For e.g., in Psa 110:1, Yahweh speaks *l'adoni* (to my lord). Per the NT, Yahweh in this passage (when 109:1, LXX is quoted/alluded to) is the person of the Father while this second lord (adoni) is the Son (e.g., Mark 12:36f; Heb 1:13; cf. Paul's midrash-like expansion in 1 Cor 15:22-28), so unless you will posit that the Father, Son, and Spirit (YHWH) spoke to a second lord who is numerically distinct from this YHWH who is the person of the Son (which is nonsense), then one cannot go down this route unless one wishes to engage in question-begging; special pleading, and rejecting the identity of indiscernibles.

 While I disagree with his Unitarian conclusions, Jaco Van Zyl, speaking of Psa 110:1 in light of texts such as Isa 45:5, was pretty spot-on in the following:

 *Trinitarians like James White argue that Yahweh (Adonai) speaks to someone else who is also Adonai. However they want to look at it, this is troublesome even to Trinitarian theology: If Yahweh is 3-in-1 God, speaking to another Adonai adds between 1 and 3 to the existing 3, leaving us with between 4 and 6 Persons in one God. If, however, you add the second Adonai to the first, then Yahweh is 2 and not 3 Persons, isn’t He (or should I say they)?* (Jaco van Zyl, "Psalm 110:1 and the Status of the Second Lord--Trinitarian Arguments Challenged," in An E-Journal from The Radical Reformation: A Testimony to Biblical Unitarianism, pp. 51-60, here, p. 60).