Your third point is valid. Just because someone is powerful, it doesn't mean they are benevolent. (I was actually trying to goad you into developing this point more, but you didn't take the bait.)
Still I think a miracle would be the most obvious proof most people would accept if someone else claimed they were the Son of God. (Rather than say reason or character.) A character test requires that you know the person fairly well, and I can't fathom how reason by itself would be acceptable at all.