Nero could have done everything Dante did, you just have to think outside the box of what the story and the game offers. I mean, we're talking about something that the creators probably never even considered for crying out loud.
Now Think about it, Nero's devil arm, he could have smashed all the weak points on the Savior with a giant ghostie fist, and after that free'd dante...though of course then Dante would have had to be the one to kill Sanctrus and save Kyrie...wich would have been awkward...at least the Dante saving Kyrie part...<< >>
Ether way, I think Nero could have done everything Dante did, finishing off the bosses, AND wielding the other weapons...only he would have provided a better explanation of WHERE he puts the Devil Arms, I mean, his arm would absorb em all right?...Dante...he just pulls em out of his ass or something??
(I am not a Dante Hater, Im just saying, the hell he puts them??)
And also the "Kyrie" trick, I doubt Nero would fall for it...moral of the Saving part, when someone you care for deeply is on the line, you can and you will do everything you can to save them.
In short, Nero would have done just fine if HE was the guy through the entire game, and note, no, Im not saying Nero should have been the only guy through the entire game...that would have gotten old way too fast >< mostly becouse of his short move list...but hey he's working with one sword and one style and one gun...when dante has 3 sword thingies and 3 diferent guns and 4 diferent styles.