When you get right down to it, it would have been impossible for a stranger's presence to go unnoticed in a small town in 1946 Sicily. Even if Michael had stayed within the walls of Don Tomassino's villa, those going in and out would have noticed. On the other hand, Sicilians were (and probably still are) acutely aware that "loose lips sink ships," especially since Michael was under the protection of the local gaboletto. So, the public wedding was not a real sacrifice of security. And anyway, note that Cal and Fab carried their luparas with them, even as they emerged from church--another nice little warning to the loose-lipped.

I always thought that more than the Thunderbolt was at work in Michael's choice of Apollonia, and to have a public wedding. After coming over to the Family ("I'm with you now, Pop"), he may have consciously or unconsciously been rediscovering and aggressively embracing his roots. Being in Sicily surely helped. By falling for a young Sicilian girl, courting Apollonia in the old-fashioned way, and having a public wedding, Michael was showing respect and welcoming acceptance of his Sicilian heritage--almost as if he were re-emphasizing that he was now "with" Vito.

Two other interesting nuances in the scene with Sr. Vitelli (one of the best scenes in the Trilogy, IMO):

--Michael freely admits to Vitelli that he's "hiding from the police of my country." Vitelli doesn't take that as a disqualification for courting his daughter--not at all. No doubt an accurate statement of a Sicilian's instinctive distrust of government, for which they'd had centuries of justification.
--Michael immediately says that the information "would be worth a lot of money...but then your daughter would lose a father instead of gaining a husband." Here we see inexperienced Michael violating one of Vito's cardinal rules: never make an unncessary threat. Later, the more experienced Michael doesn't rise to Moe Green's taunts and threats with his own--he knows what he has to do. Ditto Tom earlier with Woltz.