Assume that the scenario involving "two equally acceptable schemes of translation from a language L into a language M might carry a given sentence of L respectively into distinct sentences of M to which a single speaker of M would ascribe different truth values" is plausible .
We have an original sentence S of language L that is mapped into two different sentences S1 and S2 of language M by two different translation schema.
Then you say: "when the original sentence is presented to the native speaker of the object language L, they might either assent or dissent from it, enabling the translators to determine its truth value. Hence, if the two translations differ in truth value, there should be a way to dismiss one as incorrect while retaining the other as a valid translation."
But this is exactly the gist of Quine's thought experiment with "Gavagai" vs "Rabbit" into Ch.2 Translation and Meaning of Word and Object (1960):
A rabbit scurries by, the native says 'Gavagai', and the linguist notes down the sentence 'Rabbit' or 'Lo, a rabbit') as tentative translation. [...] For, suppose the native language includes sentences S1, S2, and S3, really translatable respectively as 'Animal', 'White', and 'Rabbit'. [...] He [the linguist] is thereupon in a position to accumulate inductive evidence for translating 'Gavagai' as the sentence 'Rabbit'. The general law for which he is assembling instances is roughly that the native will assent to 'Gavagai?' under just those stimulations under which we, if asked, would assent to 'Rabbit?'; and correspondingly for dissent.
Then the long discussion about "stimulus meaning" follows, ending in the well-know example:
For, consider 'gavagai'. Who knows but what the objects to which this term applies are not rabbits after all, but mere stages, or brief temporal segments, of rabbits? In either event the stimulus situations that prompt assent to 'Gavagai' would be the same as for 'Rabbit'. Or perhaps the objects to which 'gavagai' applies are all and sundry undetached parts of rabbits; again the stimulus meaning would register no difference. When from the sameness of stimulus meanings of 'Gavagai' and 'Rabbit' the linguist leaps to the conclusion that a gavagai is a whole enduring rabbit, he is just taking for granted that the native is enough like us to have a brief general term for rabbits and no brief general term for rabbit stages or parts.
See Dummett's comment (page 362): "As is well known, Quine's general
thesis about translation is that there are certain criteria of correct translation whose application can be determined by observation of the linguistic behavior of the speakers of the language, but that, within the
limits imposed by these criteria, many different translation schemes may
be possible between which there is no objective criterion which will select
any one as alone correct."
In conclusion: is this scenario plausible? is it useful?
Dummett's objection seems to be: Quine's theory is not a "full" theory of meaning, but just a presentation "admittedly in metaphorical form, [of] a model of language (page 362)."
See page 374: "Indeterminacy of meaning does not entail indeterminacy of translation: if an expression of L is indeterminate in meaning, then the only strictly correct translation of it into M will be by an expression which has an exactly matching indeterminacy. Of course, M may contain no such expression: but Quine and his supporters have insisted that the indeterminacy thesis is not a mere assertion of the banal observation that exact translation may sometimes be impossible".
And see page 395:
I have tried to show that, unless reinforced by the inextricability thesis, the indeterminacy thesis is quite implausible.
But it is useful to consider SEP's entry on Quine and Indeterminacy of Translation: "we need to distinguish two kinds of indeterminacy. Quine introduces the general idea of indeterminacy, in Chapter Two of Word and Object (1960), without explicitly distinguishing the two, but subsequently comes to treat them quite differently. The first is indeterminacy of reference: some sentences can be translated in more than one way, and the various versions differ in the reference that they attribute to parts of the sentence, but not in the overall net import that they attribute to the sentence as a whole. "
This seems not to be the case discussed above, because it does not affect the "overall net import that they attribute to the sentence as a whole" that must include truth value.
"The second kind of indeterminacy, which Quine sometimes refers to as holophrastic indeterminacy, is another matter. Here the claim is that there is more than one correct method of translating sentences where the two translations of a given sentence differ not merely in the meanings attributed to the sub-sentential parts of speech but also in the net import of the whole sentence. [...] This claim involves the whole language, so there are no examples, except perhaps of an exceedingly artificial kind. There is also nothing resembling a proof; in some late works, indeed, Quine refers to it as a “conjecture”."
This second kind is the one discussed by Dummett.
And see page 379-on, where the discussion shifts from "general" language to so-called theoretical terms: "It may be replied that there is no need of any elaborate argumentation to establish that an indeterminacy of translation must reflect an indeterminacy of socially agreed meaning. For, the reply may run, it is only theoretical sentences that are indeterminate with respect to translation: and the whole argument for the indeterminacy thesis rests on the premiss that even the totality of all true observation statements leaves theory underdetermined (where a statement is an ordered triple of a sentence, a place and a time, and is true if an observer would have assented to it had he been at that place and time). Hence it is assumed from the outset that, for any theoretical sentence, there can be no means agreed on by all speakers of recognising it as conclusively established."