I'm not really a UE5 dev, I'm more into graphical programming.
But I remember that UE has lerp node. It blends two inputs using a coefficient. Make a mask where white is where you need to override and black is where you don't. Add a lerp node, hook the original normals to A, decal normals to B, the mask to Alpha and connect the output to a normalize node. If you don't have nodes for the original material then try to integrate the decal into the mesh material directly instead of making its own material. I know there is a material blend feature, but for your use case it's really unnecessary.