1) I don't think the Don really trusted anyone completely outside of his sons. He wasn't worried about Brasi doing him any harm at the wedding. He likely didn't want to see Brasi because everyone knew Brasi's business. Brasi was a symbol of the uglier side of the Don's lifestyle which he probably didn't want to be reminded of on his daughter's wedding day. Brasi is like a bad tempered Rott on a short leash. FWIW, IRL supposedly the Outfit bosses similarly did not invite Sam DeStefano to many social gatherings for the same reasons ("Double deal")
Since the Don would have had veto power over the wedding guest list Brasi is there because the Don wanted him there, even if he didn't necessarily want to meet with him.
2)In book Vito shrugs and thinks that even dynamite may be exploded harmlessly if need be. In movie I don't think Vito would be the sort to get rid of people because of what they might do-especially if they were loyal to him.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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