Riva and Layra can’t be he same person, because in book 6 Riva describes her father as someone who’s crazy. In book 5 we meet Laura’s father and he seems pretty normal. So they don’t have the same parents, and therefore they can’t be the same person.
Also, Layra was born around the same time Max was, which was fifty years ago. Riva doesn’t seem to be fifty years old.