I must say first of all that the plural scenarii is clearly not the prevalent usage. To most frenchFrench speakers, it'll sound quite elitist, self-important, or snob. It doesn't refer to italianItalian plural but latinLatin, which is a sign of intellectualism here like in many other places.
For your first question : references are mainly acknoledgingacknowledging the scénarios plural, and mention scenarii as a side note. (TLFi)(wiktionnary)