Juan Felipe de Alvear y Ponce de Leon never really got the chance to participate in the government. Yes, he had a seat in the "Parliament," but he never really spoke or did anything, with the big parties and their leaders' feuding dominating politics and overshadowing whatever his fellow politicians tried to do. Eventually he returned to the military, but after the war with the Americans his troops were assigned to guard duty in the mainland in the face of growing unrest. He never got a promotion after the war. Not that he wanted one anyways given what the other generals were doing.
When the Republic was declared, he was stripped of his estate and titles and forced into the streets, losing almost everything. Only his military commission wasn't taken away, and that was what sustained him for the next ten years. Angry that the government he had sworn to protect had been overthrown and the new one was out to get him, he was among the first to join the Nationalist movement (after sending his family abroad for their own safety). Francisco gave him several medals and a promotion for his outstanding performance on the battlefield. He was among the first soldiers to enter Madrid and raise the Nationalist banners over the capital.
Under the new regime he returned to politics, with his connections to the Caudillo allowing him to finally have a say in the government. Francisco restored his titles and properties to him, making him wealthy again. His family started several businesses to help out the common people, rapidly building up a corporate empire that lasts to this day.
When Francisco died in 1975, Juan Felipe quickly switched sides, knowing what would happen next, and managed to not be prosecuted for his association with the Caudillo. He was present at the King's coronation ceremony. He died not long after that, satisfied that his family's legacy was intact.
After 1975 the Alvear y Ponce de Leon family remained a dominant force in Spanish politics and the economy. Juan Felipe's sons and grandsons were variously politicians, generals, and businessmen. His eldest son, Hernando, was Spain's first elected prime minister (and initiated a rapprochement with Argentina and other former colonies), while his second son emigrated to the Empire of Mexico and became its prime minister and his third son made millions in real estate, banking, and oil. His nephew is currently Field Marshal of the Spanish Army, and his grandson Carlos is expected to win the next general election.
From a minor noble family fleeing the chaos of Argentina's war for independence, the descendants of Carlos Maria de Alvear y Ponce de Leon had become one of the most powerful families in Spain, if not the entire Spanish speaking world.