I still think she looks like she has STB in her.
There is no defined "Mustang look" so anyone could say their grade is a Mustang. They come in all shapes, sizes and colors so there's no way of telling if she is one unless you know her background
Just about any unknown grade could look like a mustang. Mustangs have so many different breeds in their background and various looks depending on which HMA they come from. I've had three mustangs and they all looked quite different.
To me she looks a lot like a QH X standardbred cross.