It proves Sheryl thought this was the case before the ep 24 kiss scene. However, when Alto reacted emotionally to her letting him go, she knew that his feelings for her were real, which gave her the determination to kiss him, and wait for an answer. She's fairly confident of her chances, given that she's only mildly shaken when Ranka says she's not conceding yet, and even flashes the winner sign and playfully challenges Ranka to bring it on, totally different from her previous uncertain self.
Because he thought Ranka had gone mad and just betrayed humanity. Notice he never thought about her for the next episodes again until he decided to kill her. And it's Sheryl who influences him to try to save Ranka (she'd only listen to his answer if he returns with Ranka) instead of his own initiative.
Because he thought Ozma had gone mad and just up and joined Ranka in betraying humanity, and also because he's sick and tired of people endlessly telling him he's running away and criminally dense.
This is probably overanalyzing the scene, but since you bring up the arm thing, it's actually in Sheryl's favor, not Ranka's. Why? Because he's trying to take the hand of one of the girls as he approaches on his fly past with his own right hand.
Now, Ranka is holding up her left hand, while Sheryl is holding up her right. Now, if you were reaching to properly grip someone else's hand using your right hand, which hand should they be holding out to return your grip?