Let us first look at the burning of “Mance”.
It is obvious that Rattleshirt was duped and it involved many people, not just Mel. Rattleshirt was told that he was going to be executed for his crimes unless he obeyed Stannis and played his part. They were going make him look like Mance by Mel’s witchery and take him before Stannis while all the wildlings were present. Then, “Mance” was supposed to
renounce his kingship,
swear his fealty to Stannis,
pledge to keep the king’s peace and the king’s laws,
take the Lord of Light as his god.
Once Rattleshirt made this public show, Stannis would grant mercy to Mance and spare him. Likewise, Rattleshirt would get amnesty and the position of a wildling leader as well as lands to rule. This scenario is not something I make up out of thin air. All the necessary quotes will follow. Before we proceed, note that this scenario explains why Rattleshirt was very comfortable and smiling at the beginning until he saw the cage and realized that he was really going to be burned alive. Before he could confess anything and say “they tricked me”, Godry and the queen’s men assured that he could not talk. That explains “they” because they were all complicit in this scheme.
Here we see the terms I previously mentioned but there is more than that in this exchange. Stannis wanted obedient wildling chiefs to lead them for him. Despite Stannis would later tell Jon that Mance Rayder's life was forfeit by every law of the Seven Kingdoms, here we see that Stannis was ready to spare him if Mance bent the knee and met his terms. That is one of those details that shows why the popular image of Stannis as an unbending person of law is so wrong. Stannis would have publicly forgiven Mance if he did what he asked of him. Similarly, he could have legitimized Jon as a Stark and removed him from the Night’s Watch if he did what he asked of him.
Only from a single Mel chapter, we learned that Stannis spared a craven who should have been hanged and he also had sex with Mel regularly. In his world view, sparing those who break the laws encourages others to break the laws without fear of punishment. That is why he wants to be seen as unbending in the matters of law. Stannis openly bashes Ned for acting too honorably and paying it with his head. He is scornful to Jon for acting too honorable like his father. Do people think that Stannis does not practice what he preaches? As long as it is not done in an encouraging way for the public, Stannis can play with the boundaries of the law and honor as it pleases him. Whether you like it or hate it, this is how Stannis rolls.
Despite Mel’s claim, Jon pointing that the law ended at the Wall was irrelevant. Stannis already knew about that and he was already ready to spare Mance if he met his terms long before that exchange. At that time, Stannis was very frustrated because he recently got that letter of defiance from Lyanna Mormont. He complained that despite his ravens, all he got was silence and defiance from the Northern Lords except Arnolf Karstark. Therefore, he had to burn “Mance” to send a message to the world. (Again) whether you like it or hate it, this is how Stannis rolls.
Here we see further uses of keeping Mance alive from the perspective of Stannis. Previously Jon told him that Val would slit the throat of any husband Stannis forcefully makes her wed. But then she begged Stannis to spare Mance and promised to be an obedient bride. Stannis talked with not only Mance but all the other wildling leaders including Rattleshirt. He learned all he can about them from other people. We have seen him questioning Jon about Mance and Rattleshirt. Therefore, Stannis knows that Rattleshirt is a treacherous and bloodthirsty bastard who is bound to create problems. His crimes were much more unforgivable than Mance.
Yet Stannis meant to grant lands and a castle to “Rattleshirt”. It does not make sense.
It makes even less sense to give Rattleshirt to Jon as a gift, especially since Stannis planned to have the wildlings in his van, led by their own chiefs. Rattleshirt was a notorious raider. Stannis is a commander in need of soldiers and serjeants. He would never leave Rattleshirt behind, unless he knew that it was not actually Rattleshirt.
Again we come to the same point that the burning of Mance was not a matter of law but a political message to the Northern Lords and wildlings, showing how Stannis deals with turncloaks and traitors.
Finally, think from Mel’s POV. If she knew that she could cast the Mance/Rattleshirt glamor, would she not tell Stannis about it? Stannis needed to burn Mance but he also wanted to make use of him. Even if Mance refused his terms about binding the wildlings to his cause, Stannis would still want to use him for his knowledge about the Others if not for anything else. If Mel knew that she could provide that to Stannis with her powers, why would she keep it as a secret? It does not make any sense. If Mel finds a way to help Stannis, she makes sure that Stannis knows about it (especially her sorcery skills). That is her only way to convince Stannis of her power and in return gain his confidence. Mel needs to act this way in order to keep Stannis dependent on her. If she kept all her sorcery a secret, how could she hope to convince Stannis to burn Edric?
Moreover, Mel working behind the back of Stannis in such a critical matter does not seem compatible with how their relationship is framed. Mel knows that Stannis will use her as long as she displays power, yet at the same time, Stannis doesn't forget any betrayal. If Mel was capable of doing such acts behind the back of Stannis for the greater good, she would have burned Edric by herself and only after the fact tried to convince Stannis with the dragon she expected to awaken.