They are just starting the relationship, both feel pressured about messing things up, then he goes and hide something from her and she doesn't want to retaliate because she cares for him, it's not just stupid, it's teenage drama, dumb, but it really happens.
Sabnock is such a bro, he not only would accept Iruma as a brother, but also made sure to make Sylvia say how much she wants to paint no matter the risks.
More like it never got to be used, so it's like a failed weapon of sorts. The insignia is the maiden's mark, being Meidena's blessing from their god, so it's saying they have god's protection and don't need these weapons.
Now even the dad is growing up to Iruma, no good dad can fight the pure smile of joy in his daughter's face, specially if it reminds them of her mother. Next episode is going to be so damn adorable.