Good analysis, Ryll, but I'm not sure it's entirely correct. It's not like her maturity, or like you said, her transparently pretended maturity, was all fake. A good deal of it stems from her concentration and desire to reach the goal. People with a vision and the willpower to go for it automatically gain some charisma. Compared to the likes of Kumiko living real adolescence, like wanting to switch away from her instrument of many years just because, Reina certainly would score some adult points. The fact she could pull off such a long act in front of Kumiko also speaks highly of her mental fortitude, even if not maturity directly.

Shinta's single-minded obsession with girls' armpits doesn't suggest high maturity, though.