I realize all of that about Sonny not being able to stay couped up, about his needing to see Lucy, etc.
But the real point here is that he was negligent in that he would put his sister in such a vulnerable position during a time of war. Think about it, Sonny even wants to send Michael, a 'civilian' at the time, over to Luca's place and I believe that it is Tom who suggests that they should keep Mikey out of it.
Yes, but I think the reason they wanted to keep Mikey out of it was not just to protect him but also to make it clear to everyone (in line with the Don's wishes) that Michael was a non-combatant, not a criminal and off limits. Showing up with thugs at Luca's place asking questions would have alerted Tattaglia and others that Mike was a player on the board.
As a woman, in that time and place no one would have mistaken Connie for anything other than a civilian. It is even possible that had hitters been able to surprise Sonny with Connie in the car they MIGHT have waited to get him alone.
As late as the seventies and eighties it was considered a serious breach of mob etiquette to deal with an enemy in front of his women (wife, mother, sister). The events in Philadelphia relaxed this taboo.
If the Corleones had given Carlo a house on the mall perhaps with Sonny literally next door, Don's orders or not, he wouldn't have been so quick to lay his hands on Connie. And just possibly if the Family had given Carlo something more important (and yet completely removed from any real Family business) he wouldn't have felt the need to take his frustrations out on his wife.
It's fun to speculate.
But in any event Carlo lived where he lived and did what he did. It was necessary to move the story along and show the critical flaw in Sonny's nature.
