You can get all the support you want from 'Board Attorneys'.
If we are thinking of this as simply a matter of law, then yes, by masterminding the massacre with Michael he was just as guilty as those who pulled the trigger.
The law is not what has been in question, and it was obviously of no concern to those characters involved. What has been discussed here is whether or not Vito Corleone broke the peace he made with the other Dons, in order to end the war and get Michael home.
And he did not break the peace.
Apple