How can someone do something "in the person of Christ" and not be an alter Christus?
Exactly, the real translation of the meaning of alter Christus is in the person of Christ, not "another Christ" as some loons, liars and the genuinely misinformed would have you believe.
Because elsewhere in the Bible Jesus tells us not to believe men who call themselves "another Christ," and Paul teaches that there is "only one God and one mediator between God and men, the man, Christ Jesus."
So clearly Paul does not mean men are to become an "alter Christus." He means we are to become Christ-like. To forgive as we are forgiven. To follow Christ's example as we live and forgive others.
Rome takes the simple words of Scriptures and inflates them to blasphemous proportions.
You and I are called to live our entire lives "in the person of Christ" because as Christians we are indwelled by the Holy Spirit who motivates us to trust, obey and believe.