Nice ideas! But I think a little diferently. I think that the cannon was rose's secondary weapon because she was leader and all. Rose must've transformed into the cannon to protect steven from the red eye. Maybe?!?
The light that came from the cannon was rose quartz because she was trying to save the gems which are a.k.a kinda part of her family heck she left them in charge of her son because she knew her husband couldn't teach him how to use his powers. And she probly wanted to leave leadership to steven when he learns to controll his powersfully and he knows the other gems would have taught him how to be a great leader