Personally I think Sumika was made to be the central pivotal of the entire alterverse from the beginning, no matter in Unlimited or Alternative or any of the side stories,
Spoiler for Desu~ Ex Sumika:
since she is the single most important resource for the plan. Essentially she was created to be the Desu Ex Sumika for the series, the point on which the path of the world branches. That was her purpose in the trilogy as written by Kouki, and it was as well played as Luke finding out he was smooching his Twin sister.
I like the way they bought Sumika's character and existence into play as a contrast with how the MC interacts with the others. Instead of people that have a relatively sheltered life in that world (In extra you dont even have to see BETA...) You have a character who has really seen and bathed in shit, and still somehow survives... holding on to the one thing precious to her so hard it causes a quantum breakthrough of time and space.
This is the one character who holds her feelings hard enough to make Alternative possible. Can one really say that other characters as they are (as classmates/ friends only) going through the same events can have the same effect? Can their relationship with the MC at base (at the start of the VN) be as strong as what Sumika holds? Are they as sure of their feelings that without the interaction of the VN events they would hold him foremost in their minds? That is what Sumika has done, since she was out of the picture from the start, while the others got to know him from the time of the transition only. Even in the end of Unlimited, Sumika is still in his thoughts as he says goodbye to the girl he ends up with. That is how critical to the MC she is.
This is what I feel about Sumika, and yes, she was scripted to overshadow the others in terms of importance. Alternative has always meant to bring closure to the series, and to do that someone must be chosen. She was picked from the start by the writers, and thus suffered the most in story as well.
This is my own feelings. Even the million euro man recognizes her as the single most courageous person without knowing her name.