@random I don't think Dia's a time traveler here. I'm thinking it's just the drawing style that's making Nico and Ruby look younger since Dia still looks about the same age as she does in canon.
Not sure about that. I do know they have a rivalry going on between each other which naturally makes them popular to ship together. Same reason that Asuka and Homura is popular too (probably anyway).
What are Dia's ships though? All I know of is Kanan, Mari, Chika, and Hanamaru while Riko has You, Chika, Kanan, Yoshiko, Maki*, and a few others I can't think of right now.