Why would I assume that God exists as some type of polypersonal being that is never once taught? I'd rather take Scripture for what it says and apply principles that are explicitly shown in Scripture to understand them.
A) Actually, it was an accepted way of writing to attribute to oneself what is said/done to ones agent, for one's agent is as himself, as the Talmud clearly defines. So if God's agent was pierced, according to understanding of the day, it would be the same thing as saying that they pierced the one that he sent.
B) Well this goes back to the principle of agency. To pierce the agent is to pierce the sender... and so the agent is indeed pierced. There is no issue.
C) Well again, agency would apply quite naturally. In case you do not understand this concept, Johnson explains it: "In Hebrew thought a patriarch’s personality extended throughout his entire household to his wives, his sons and their wives, his daughters, servants in his household and even in some sense his property. The "one" personality was present in the "many" who were with him. In a specialized sense when the patriarch as lord of his household deputized his trusted servant as his malak (i.e. his messenger or angel) the man was endowed with the authority and resources of his lord to represent him fully and transact business in his name. In Semitic thought this messenger-representative was conceived of as being personally - and in his very words - the presence of the sender."
In other words, to attribute something or do something to the agent is it attribute it or do it to the one that sent him. Buchanan brings to light this application to Jesus: "As apostle or agent he was sent with the full authority of the one who sent him. A man's agent is like the man himself, not physically, but legally. He has power of attorney for the one who sent him. . . He has the authority of an ambassador who speaks in behalf of a king in negotiating for his country (Ber. 5:5). Jesus said that the one who received his apostles whom he had sent received Jesus himself, and not only Jesus, but the one who had sent him. (John 13:20). . . Legally Jesus was identical with the Father, but physically the Father was greater. . . As an ambassador or apostle, the Son has authority over everything since he is given legal authority and is supported in everything he does "by the word of [God's] power." He speaks for the One who sent him."
Buchanan notes John 13:20, which tells us that he who receives the Son receives the Father, and so in a similar way, what one does to the son is what one does to the Father.
If you want to argue for a polypersonal God, you have to prove that one exists first. Otherwise, what you are doing is purely circular, expecting us to interpret Scripture in light of there being one when you have not yet demonstrated that one even exists!
Mondo