Steve,
It's my guess that Felice Santangelo was an abandoned baby. On the marriage registration is says "Felice Esposto Sant'Angelo."
Yes, that Esposto would suggest he (or some of his paternal ancestors) was not the son of a married couple. Having said that, I have noticed other times the second surname added, and then years later the child being legitimised by the parents through a late marriage loosing that Esposto in later life. This doesn't generally happens when the surname doesn't have the second last name.
About the Santangelo, there are 3 main branches in west Sicily. Near Palermo in the town of Belmonte Mezzagno, then just in front of the Sea of Sicily in Sciacca, and near by in Castelvetrano. You'll find Santangelo also in Palermo, Trapani, Alcamo, Mazara and spread thin on the communities of the Belìci Valley and of the Madonie mountains, but take in account that Sciacca will account for half of Western Sicilian Santangelo.
The Sicilian town with probably the highest concentration of Santangelo is on the other side of the island, in Adrano. The province of Catania accounts for half of all Sicilian Santangelo.
By a weird combination, the city of Trapani in the current italian legislature is represented by an unprecedented number of senators, 3. One of these senators is Vincenzo Maurizio Santangelo.
This is Mr Santangelo's page on the Italian senate website:
http://www.senato.it/leg/17/BGT/Schede/Attsen/00029174.htmThis is Mr Santangelo's personal website:
http://www.vincenzomauriziosantangelo.it/The chap is quite approachable, you may try to contact him and ask if someone in his family has performed genealogical investigations, and if so, if they could share those with you.