Joy, I will try and answer your questions as best as I can although I too have gaps in my knowledge of del Fierro-Ferriols-Magsaysay trees.
Vicente aka Macario Ferriols was my grandmother's grandfather. My grandmother was Amalia Corpus married to Ambrosio Magsaysay and daughter of Ramona Ferriols, the brother of Lolo Don Pepe Ferriols whom you spoke of.
My aunt, Anita Magsaysay Ho(the painter), writes is her published autobiography that Don Fernando was a Spanish admiral and that he had a son or grandson named Miguel who became governor general. She further wrote that Miguel had 13 children including of course the three sisters Ysabel, Lucia, and Margarita (aka Mariquita). She says that among the siblings was Serafin from whom President Magsaysay's mother was descended from.
While it would be nice to think that we are both descended from a great Spanish admiral, I am at this point inclined to believe perhaps Fernando and Miguel were one the same person and the former was a less used middle name. It was Miguel who left the Spanish navy as a marine lieutenant to settle in the Philippines and was appointed in 1809 as alcalde mayor of Zambales.
It is also mentioned that in Samar he held a similar post which most likely was before his Zambales appointment, the former being much smaller in size and population then. A relative who has Lucia's birth certificate says she was born to Miguel del Fierro and Sebastiana del los Reyes in Catbalogan in 1816. Now as to why he would maintain a household in Catbalogan while holding office in Iba is a question that needs to be asked. It appears though that Lucia, Ysabel, Marquita, Manuel, and most of the other siblings grew up and died in Zambales.
Just how Lola Pitang, President Magsaysay's mother, is descended from Serafin del Fierro is another gap that remains unanswered. I would think this is a question that should be asked of her surviving children or grandchildren as it there seems nothing on the subject in published accounts. She apparently had a brother, Juan Crisostomo (Lolo Tomoy) as well as other sibling sisters.
I too have Vicente Ferriol's biographical notes in Spanish and have had it translated to English. I have the original Pilipino document from which the Venzon site's del Fierro document was based on. That document was supposedly written by my uncle Jesus Magsaysay, the son of Exequiel and Perfecta. I think it strange that my Tito Jesus would neglect tracing out his mother's del Fierro lineage unless it was his intent not to.