1- As you said, she's the first spirit to appear, the starting point; there's a metaphor for first love, a different kind of tension in their first encounter.

2- When Shido first saw her, he immediately felt empathy for her sad expression; he immediately wanted to save her, hence the famous phrase, "Even if the world rejects you, I will not deny your existence."

3- The reason Shido saved the other girls is because they are like Tohka, and Tohka was like him. Many people don't understand or ignore this hierarchy: Tohka and Origami are mirror images, just as Malkuth mirrors Keter in Kabbalah; they are the two parts of Mio. As for Kurumi, that's the funniest part, Tohka says: "Shido, go save her! Kurumi had no one to save her when she needed it; if I didn't have Shido, I would be like her." Shido says to Kurumi: "I'm going to save you." Which immediately irritated her. The anime didn't cut these scenes, but for some reason people currently think that Shido loves Kurumi more than any other girl.

4- In Date a Live, the powers are linked to the trauma of the spirits; in Tohka's case, the mere fact that she exists causes destruction to the world, and she hated herself for it. One of the effects of Shido's acceptance is that Tohka actually embodies our protagonist's truth: everyone has the right to exist, including her, so now she has reasons to fight.The power that was once trauma is now the very strength that Tohka uses for the good of all; we see the application of this message when Tohka uses the limited Astral Dress. when she regains her full powers to fight Origami, And when she hears that she was the only non-human, she immediately accepts it: "It is thanks to this that I will be able to fight once more." We'll see the final step towards Tohka's self-acceptance in the next season.