Sorry Chica, other than personal anecdotes I have nothing to offer: My Mother-in-law (from Madrid) lived with us in the U.S. for about 25-years and we (daughter, son) had to (and continue to) speak
Castellano at home.
Daughter visits Spain often: has been taken for Madrilenha
In Andalucia, people have asked me if I was from Cantabria (where I studied Castellano). I recommend UIMP
Universidad International de Menendez Pelayo in Santander for someone who is seriously studying
Castellano.