Ok to start, yes this new boy's name is not Syaoran. Yuuko mentions that he does not give his "true name" thus implying that his name isn't Syaoran. Later on you see that his mother (whom Yuuko says is Sakura) has paid the price with the star wand showing that his mom is CCS Sakura. So basically, this "Syaoran" is the kid of CCS Sakura and Syaoran.
Kurogane cut off his arm because when Celes was being pretty much destroyed, he and Fai were being trapped in that little ball. Mokona's earing opened the ball somewhat but only Kurogane could escape. So, he cut off his arm and left Souhi in the ball so that he could take Fai out as well.
If you need more explanations just say.
