I think Curley’s wife was trying to impress Lennie and to give him the impression that she was unhappy with Curley and wanted more out of life and marriage. Therefore she was leading Lennie on in a way that wasn’t flirting.
I think George had to kill Lennie because if I was George I wouldn’t want some guy to shoot him who hates him…I would want my best friend to shoot me and put me to peace. And yes I feel like there is a connection between Lennie and Candy’s old dog because they both had to shoot there best friend.