In designing an artificial dialect of the Greek Language, most words and inflections have predictable patterns with a well-guessed/documented evolution which show the origin of the word's roots and stems. This makes it fairly easy to go back in time, change the evolution of the sounds or orthography, and use an existing vocabulary from the already existing Greek Language to create an artificial dialect.
For example, I could thus look at the dialectical declension of a word like σύ, and see that in the Genitive, Attic renders it σοῦ, Ionic σεῦ and others σέο, and infer that the root/stem is σε-, and that the contraction rules show that the irregular declension is σε-ο, then insert my own sound changes based off my own set of rules. Likewise I can find out that σφᾶς is a normal contraction of σφέας or σφέανς with an irregular enclitic of σφε.
What I can't seem to do is take a root that is only used in the dual and transfer it to the plural. In the rare First Person dual, the pronouns used are (NA) νώ and (DG) νῷν. I want to make a plural pronoun using that root in the manner of Latin "nos" instead of ἡμεῖς (which possibly reconstructs from ἀσμέες or the likes.) I can conjecture several possibilities based off a pattern, like (NADG) νῷ, νώς, νῷς, νῶν. But these have no real basis in the Greek evolution from its first ancestors to Classical Greek. I can't use σφεῖς or σφώ or σφωέ to trace a pattern without a good reason, because all of them seem to have their patterns traced from the evolution of ὑμεῖς, σφι, and νώ. Where do irregular declensions come from, and how are they most authentically reproduced in a constructed dialect? I am not a professional, so please give me some leeway if I have made errors presenting this question.