Considering the OT passages that might be the origin of Melito's thinking (if he is not working from a haggadah, with which he would have been familiar with from his Jewish upbringing), there may not be too much difference in his understanding of "ὑμεῖς δὲ ἔσεσθέ μοι βασιλείαν / ἱεράτευμα καινὸν / λαὸν περιούσιον / etc." and "ποιήσω δὲ ἡμᾶς βασιλείαν / ἱεράτευμα καινὸν / λαὸν περιούσιον / etc."Peter Streitenberger wrote:
[καὶ ποιήσας ἡμᾶς ἱεράτευμα καινὸν
καὶ λαὸν περιούσιον αἰώνιον.] (495)
Another thing, and perhaps there is a skew in my thinking because of the limited evidence that we've discussed so far, is that it seems that this construction - using a plural subject with a singular class is "Biblical" in expression. The passage about choosing a king from Aristotle is talking about a singular king. In these other Christian examples, I'm wondering if the Greek is stretched. The other parts of that short excerpt (which contains very simple grammar) don't really indicate that he was working with Greek influenced by his reading the OT in Hebrew, but that is not an easy thing to test.