1. When people cry "with one voice" four paragraphs worth of stuff (5:2-5), it isn't spontaneous. They're being led through covenant-making language, which is how Benjamin responds to it (5:6ff).
2. Taking upon them the name of Christ is a priestly image. The plate on the front of the high priest's turban bears the inscription QODESH L'YHWH, "Holiness to the Lord", so the high priest literally had the name of Christ, YHWH, upon him. I think Benjamin's repeated reference to this "head" in the verse is an allusion to this. Benjamin is turning the people of Zarahemla into a nation of priests.