Maria Teresa an Fernando began their relationship in 1904, during a visit of the family to Spain. Although Infanta Paz and her family lived in Munich, they often visited Spain. Actually, Prince Ferdinand was born in Madrid. They wanted to announce their engagement quickly, but due to the deaths of Queen Isabel and then the Princess of Asturias, it was postponed to 1905.
So, answering your question, no, she was never considered to be Carlosís second wife.
About Alfonso and her cousin Pilar, no, either. His marriage was a political question and a British Princess was always the first choice. An Austrian bride or a German one were also considered. But the Kaiserís candidate was a Mecklenburg. .
Was she touted as a potential queen consort of King Alfonso XIII of Spain, a first cousin? If so, there would have been a double marriage between their families, since her brother Ferdinand married his sister Maria Teresa in 1906. That union was the third straight generation of intermarriages between the Bourbons and Wittelsbachs, since both his mother and paternal grandmother had been born Spanish princesses.
Speaking of which: was Infanta Maria Teresa ever thought of as a potential second wife to her widowed brother-in-law, Infante Don Carlos of Spain (born a prince of Bourbon-Two Sicilies)? Her older sister, Infanta Maria de las Mercedes, had died tragically of complications from childbirth in 1904.
After all, unlike the United Kingdom, which legalized the remarriage of a widower to a sister of his deceased wife only in 1907, Spain and Portugal have always allowed such unions. Otherwise, there would have been no way that Infanta Maria of Aragon (third daughter of the famous Catholic monarchs, Ferdinand and Isabella) could have married her widowed brother-in-law, King Manuel I of Portugal, after her oldest sister also died in childbirth.