What is the ruling on saying "Were it not for the grace of Allah and the grace of so-and-so" or "Thanks to Allah and thanks to so-and-so"? And what is the most appropriate phrasing for thanking someone and attributing credit to them, while emphasizing that all grace belongs to Allah?

Using the conjunction waw (و) to connect the act of a servant with the act of Allah the Almighty is not permissible, because it is a minor shirk (associating partners with Allah) that implies partnership. The correct approach is to use (thumma - ثم), which implies sequence and a delay, such as saying: "Were it not for Allah and then so-and-so."