I think it would be a really cool retcon if jaina, sylvanas, garrosh, and varian were all considered to be in the actual fight with arthas. And the consider the aftermath:

Garrosh stands over the fallen Arthas and lets out an orcish howl of victory

Sylvanas approaches the fallen Arthas and says something like "I have my revenge - now die like the foul prince you once were"

Varian steps back stoicly and says something to Tirion like "It is done," then turns to Garrosh and says "With this victory, we will recall our forces from Northrend. We will have no further quarrel with you here," and then walks off.

Jaina, it is only fitting that her magic give the killing blow - it would considered her frostbolt or fireball or pyroblast or whatever that delivered the final blow. After delivering the final magic missile, she collapses near Tirion, crying.

Tirion, watching everything, helps Jaina up and as she's sobbing on his shoulder, says something like "Be at peace. I know this is a bitter moment, but you have triumphed over your tragedy today." As Varian is walking away, Tirion helps Jaina over to him, and Varian leads her away while Garrosh and Sylvanas also walk into a sunsetting type scene.

After that, Tirion watches the scene between Arthas and Terenas, and the scene with bolvar plays out.

That's how I remember it anyway :)