No one would have been stoned as this was all dramaitzed theater for effect. No one had the right to carry out capital punishment but Rome. This was nothing more than a test of Jesus' knowledge of the law. There was no danger of anyone getting stoned.
I wonder if the lady, taken in adultery, would have been equally relaxed about the possible consequences of her public referral. And I wonder what the follow-up would have been had Jesus simply said: "Have her stoned."
No need to wonder when looking at what the texts state. Let's take a look:
And early in the morning he came again into the temple, and all the people came unto him; and he sat down, and taught them.
Here's the problem. The keepers of the law are losing their status to this Johnny come lately who is grabbing all the attention. This isn't good for business. Notice that the author says "all" the people are listening to him.
3 And the scribes and Pharisees brought unto him a woman taken in adultery; and when they had set her in the midst,
Again, this is pure theater. Jesus is being tested for his knowledge of the law.
4 They say unto him, Master, this woman was taken in adultery, in the very act.
5 Now Moses in the law commanded us, that such should be stoned: but what sayest thou?
Verse 5 is key to understanding that this is a test of his knowledge. They bring this woman and suggest that she should be stoned, but they never state exactly what they mean by "such". They suggest that "such" should be stoned, but the Mosaic law provides more than one case for adultery. The scribes and Pharisees haven't indicated whether this woman is married or single. The other party necessary for this offence to occur is also conspicuously absent which has puzzled those who have no reason to be tested on their knowledge of the Mosaic law in the first place. For those who do warrant this attention, a test is required, and this is precisely what the texts state.
6 This they said, testing him, that they might have to accuse him.
Accuse him of what? Breaking the law? Nope. He's not the one in the dock. He's the one being tested remember? He's the one who has "all" the people listening to him teach so he's the one they want to accuse of not knowing what he's talking about.
But Jesus stooped down, and with his finger wrote on the ground, as though he heard them not.
He wrote on the ground. Here again it would never enter into the scribes and Pharisees heads to bother testing someone who knows practically nothing about the Mosaic law. However, their hubris is in assuming that their test subject knows less than them, and the power of this scene is in this setting. A packed Temple. This would normally be a good thing for business, but not this time.
The scribes and Pharisees are presenting Jesus with a law that has fallen into disuse due to the fact that God left the Temple to be destroyed; hence the renovation project going on at that very moment, and the ability for rocks to be picked up in the first place(also as props).
Normally both parties would be brought to justice(Deut.22:23,24), but in the case presented "such" was not the case when a married woman who had been warned not to seclude herself with other men(Num.5:11-31). This was for at least two reasons. If the wife was found innocent the other party needn't be exposed, and out of respect for the husband. That procedure required curses be written out, hence the writing in the dirt . This is all theater, and secondary to the fact that the author's of John's gospel are spotlighting that none of this would be happening unless God were in the temple in the first place, but that's just the thing they're spotlighting with Jesus' knowledge of the law. Jesus is the high priest carrying out righteous judgement, and he's doing this because God is back in the Temple. The scribes and Pharisees see that he has ruled correctly and his knowledge is beyond question. God is the only one who is without sin. Therefore...
7 So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.
8 And again he stooped down, and wrote on the ground.
9 And they which heard it, being convicted by their own conscience, went out one by one, beginning at the eldest, even unto the last: and Jesus was left alone, and the woman standing in the midst.
It is also pertinent to note that these narratives were developed within the Jewish liturgy. They are a reflection of the birth and development of the early church, and their struggles with the status quo. The woman is a type for Israel as well as the church in the New Testament. This theme would have been in the forefront of the early church so it should come as no surprise that it shows up in their liturgy.
10 When Jesus had lifted up himself, and saw none but the woman, he said unto her, Woman, where are those thine accusers? hath no man condemned thee?
11 She said, No man, Lord. And Jesus said unto her, Neither do I condemn thee: go, and sin no more.
12 Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.
I don't share your view that the Romans concerned themselves too much with one barbarian killing another. They had a touch of multiculturalism and laissez faire, as long as it didn't interfere with Rome.
And usurping Roman power was of no little concern to Rome, especially when dealing with a people who could erupt into a mass riot over the killing of one barbarian by another one. They would't have placed a tyrannical ruthless despot in charge of a people who were meek and compliant. They put him there to make sure they didn't think about getting out of line. The fact that the texts point out that Jesus was found innocent and tortured, then crucified by the Roman government spotlights the fact that they had no problem making sure no one else got any bright ideas to take matters into their own hands. Rome would decide who would die, even if Rome considered them innocent.