Interchangeability of nominative and genitive forms of Spanish patronymics?

I’m thinking of examples like Álvarez/Álvaro, Alves/ Alves, Benítez/Benito, Díaz/Diego, Domínguez/Domingo, Fernández/Fernando, Giménez/Ximeno, Gómez/Guillermo, González/Gonzalo, Gutiérrez/Gutierre, Henríquez/Henrique, Ibáñez/Juan, Juánez/Juan, López/Lope, Márquez/Marco, Martínez/Martín, Menéndez/Menendo, Muñoz/Muño, Núñez/Nuño, Ordóñez/Ordoño, Ortiz/Ortún, Peláez/Pelayo, Pérez/Pere, Ramírez/Ramiro, Rodríguez/Rodrigo, Ruiz/Ruy, Sánchez/Sancho, Suárez/Suero, Vázquez/Vasco, Velázquez/Velasco.

Langston Hughes in Spain

Idealism vs realism as a Stalinist hack copes with the Italian bombing of Barcelona but struggles to explain why Moorish peasants are slaughtering Spanish workers in the battle for Madrid.

Langston Hughes in 1936.

An old Dutch A&E joke

And the abbot who thinks little of his neighbour the emperor’s worldly ambitions.

How Emperor Charles V ended up talking German to his horse (1)

Domain-based code-switching from Daniel Bomberg’s Jerusalem Talmud to Hieronymus Fabricius’ De locutione. Featuring the wit and wisdom of Rabbi Jonathan of Beth Gubrin, Padua’s medical school and Jewry, and the Polish utopian Jan Zamoyski. With excerpts from Fellini’s I Clowns and a bodice-ripper by Kent M Chater, whose Agent Alighieri claims that “Like the great Emperor Charles V of the Holy Roman Empire and King of Spain I speak Spanish with God, French with men, German to my horse, and Italian to the ladies.”

Charles V and his horse discussing sociolinguistics before the Battle of Mühlberg (Titian <a href='http://en.wikipedia.org/wiki/File:Carlos_V_en_M%C3%BChlberg,_by_Titian,_from_Prado_in_Google_Earth.jpg'>@Prado</a>).

Counting the corrupt

A numerate, corrupt mayor, some innumerate anti-corruption campaigners, and de Tocqueville.

100% of Spanish graduates interviewed (sample size: 1) said this <a href='http://commons.wikimedia.org/wiki/File:Rechentisch.png'>abacus</a> was in fact architect's plans for a bowling alley.