Regarding the egg whites, am I basically making a sugar free meringue?

That's a particularly interesting question because I've never used egg whites in my Tira Mi Su. And I learned it from a native italian, so I know it's "authentic." ;-)

You're whipping the whites to stiff peaks in order to lighten the mascarpone filling.

By definition, a meringue is egg whites and sugar, so it's not a meringue without either one. Just call it whipped egg whites.

In the same manner, you wouldn't call a bowl of sugar an egg-free meringue.