In the season 2 arc about Francesca going back to Italy, why was Violetta more affected by the behaviour of Francesca's father than Francesca's other friends? Of course, this is easy to attribute to her being Francesca's best friend. However, in episode 51, when Violetta talks to Francesca's father, it becomes clear from her words that she has realised Carlo was treating his daughter the very same way her own father used to treat her in season 1. The video and the song are an attempt to recreate the effect of the season 1 finale for Francesca so her father would let her stay.
Diego is definitely not a Suspiciously Similar Substitute for Tomás in season 2 - León is. In the beginning of the series, he is a bad boy sort of character, part of Ludmila's crew. Even after he does a Heel–Face Turn, he is still the more aggressive and brooding one of the two guys in the love triangle. In season 2, he is much softer and more sensitive, making room for Diego to be the new aggressive bad boy, who also goes through a Heel–Face Turn by the end of the season.
Diego himself is a Suspiciously Similar Substitute - for León. As explained above, their dynamic mirrors that of León and Tomás in season 1, with Diego taking over León's role in it.
Why would the writers originally plan on having Violetta and Tomás as the Official Couple before the ending of season 1 was rewritten? Pablo Espinosa, the actor who plays Tomás, is billed third in the opening sequence after Martina Stoessel, which would also serve as a clear evidence of this.
Because of this, Diego wasn't supposed to be introduced in season 2. However, as Espinosa decided to depart the series after season 1, he was added to the show, as a Suspiciously Similar Substitute to León, to strengthen the love triangle Violetta previously had in season 1.
