Could I2 be used instead of Cl2 to test for Br-? Why?

No. Chlorine is the strongest oxidizing agent of the halogens, Iodide is the weakest. Conversely, Iodide is the strongest reducing agent of the halogens, and chloride the weakest. Bromine and bromide are midway in both series. Therefore Cl2 oxidizes Br- and I2 does not. Conversely, I- reduces Br2 and Cl- does not.