Actually Magellan did not leave any descendants in the Phils. before he was killed.
Many of the Filipinos had no surnames so they had to take one.What the Spanish gov't. did was assign surnames from a book.Magallanes was one of those names given and in Argao, Cebu where my grandmother was from (Cebu being the place where Magellan landed anyway) many of the Magallanes family married into my grandmother's families.They were all not descended from Fernão Magalhaes.
The population of Spaniards in the Phils. during its most busiest time was less than 1% because of the distance and the time to get to the Phils.So the Spanish influence as far as blood line goes wasn't significant at all.