No matter what the Royal Academy of the Spanish Language says, and most of the books you can read, Galician-Portuguese is the mother Language of Spanish. Adding that the three-branch separation alluded (which occurred between the XI and XII centuries) was definitive.
This is the historic order that no one can change it anymore: Latin > Galician-Portuguese > Castilian > Spanish.
So similar written, so different spoken. Why? The main difference relies in the accents and pronunciation on both languages. If you are reading a Portuguese text with Spanish accent and pronunciation, most of Spanish persons will understand you more and much better than if a Portuguese person reads the same text with Portuguese accent. Between Portuguese and Spanish there are just as many similarities as there are differences between them. So speaking one language does not mean you can speak the other;
And what exactly is Portuñol?
For a Portuguese speaking person, basically speaking like you normally would but throwing in whatever Spanish words you know and hoping to God that you’ll be understood.
Most Brazilians that not speak a word in Spanish should managed to get by with Portuguese only because the many words that is similar like “Casa”. Lots of Brazilians can understand Spanish and South Americans can understand Portuguese if the other speaking not so fast. For last, Spanish speaking people from South America have an easier time understanding Brazilian Portuguese, while people from Spain deal better with Portuguese from Portugal.