It is not abnormal for the entire populace of a small Italian town to attend the wedding of one of their citizens. To most in the village, it was just another wedding. Maybe it caused some curiosity that he was an American but I highly doubt that word reached out of the town, especially considering that the villages are placed so far apart and citizens do not leave the town very often at all.

Even if word did stretch farther then the walls of Corleone, it wouldn't go far and there would be few people of signifigance that would even care.

Besides, Michael was always protected by his bodyguards and the power of both Don Tommasino and Don Vito combined.

It was impossible for Michael to expect that his bodyguard would be in on it. My guess, is that Fabrizzio was in on it from the beginning that his job was to kill him.