Am I applying the KCL the right way?
the initial equation should be:

$$\frac{Vs-Vin}{Rs}+\frac{Vo-Vin}{Rf}=\frac{Vin}{R}$$

assuming Rf is large, this simplifies to:

$$\frac{Vs-Vin}{Rs}=\frac{Vin}{R}$$

$$\frac{Vs}{Rs}-\frac{Vin}{Rs}=\frac{Vin}{R}$$

$$R Vs - R Vin = Rs Vin$$

$$R Vs = R Vin + Rs Vin = (Rs+R) Vin$$

$$\frac{Vin}{Vs}=\frac{R}{R+Rs}$$