Am I really that predictable with her actions?
Maybe, maybe not. Can't say I know Sapphire too well beyond her recent actions, and we both have already revealed our characters true nature at some point. Gren would certainly catch wind of what Shiro is going to be doing, and he'll figure that Sapphire wouldn't want Shiro anywhere near it for the simple reason that she doesn't want Shiro near it. But Sapphire isn't exactly a sentimental person, but she is pragmatic. It would be in her character to simply go and make a new weapon for her instead of trying to get her old ones, at least if the cost of producing them isn't too much. Gren will just ensure that it happens, even if she tries to do otherwise.
Of course even if she does catch wind of Shiro getting her weapons, she'd still likely make new ones anyways in the event he fails. Or more likely, in the event she rejects Shiro's efforts during her rare moment of acting on her emotions.