Genea is polysemous. So what?
You should go with the meaning that the scripture supports instead of cherry picking one that fits your presupposition. That's what.
So what? Genea may have shades of meaning depending on context, & these meanings derive from the etymology - it's not a good example of polysemous. You could try cleave - asunder or to one's spouse. As Kyredneck says, your choice of meanings is more derived from your presuppositions than the general meaning of the words.
As for parousia, the meanings are closely related - when Jesus comes he has arrived & is present. The way the word is used, in its context, has resulted in doctrine being developed.
Words have their meaning, sometimes shades of meaning, but the context must be the first consideration, rather than one's theology.