Ahhhh okay, I follow! So this bit is an issue of no distinction between plural vs singular:
Maybe something like....?
"Riaj'nud thanks the eldest, and tells them [...]"
And then this bit an issue of unclear antecedent, where "they" could refer to anyone in the room:
"Riaj'nud thanks the eldest, and tells them why they have come. 'I was called for [...]'."
"Riaj'nud thanks them for the recognition, telling the eldest that they are here because they had been called for."
Plus in context, it's hopefully clear that Riaj'nud was the one that's called, outside of this single sentence -- so like, you could TECHNICALLY interpret that last one as "telling the eldest that the three of them are here because the three of them had been called" but in context of the story it wouldn't make any sense, so you wouldn't interpret it that way.
I keep thinking of an odd, short book we had to read in highschool called Anthem, where the shtick was that because the people in this dystopia wanted to smother the concept of individuality, singular pronouns had been forbidden -- they only used "they" and "we." At first this throws you off in a few places, but when the whole book is written that way, you get used to it, and start assuming that "they" and "we" are usually gonna be singular words.
I feel like it'd be similar here? Like you'd want to be very careful initially, but after a while, readers will get used to not assuming "they" is plural when it makes more sense as singular in a particular sentence.
Alternately, you could go with an established gender neutral pronoun like xe/xir or ey/em/eirs to have a way to distinguish more easily between plural and singular.