In my opinion -
The fatal assault on Arthur, with intention, meets the definition of murder.
The injuries to his brain and spinal cord, and the blunt force trauma, as well as the bleeding in his brain which showed a day or two of healing, caused by salt poisoning, and restricted water intake, ignoring his requests for a doctor when he was suffering with headache through his last night on the concrete floor - and instead ramping up the violence and pouring salt down him - showed intent to cause death. (IMO) Intent to hasten an end to the problem. There was no future for Arthur unless they stopped but neither of them was prepared to stop because that would require self-recrimination (not present) and agreement, or TH to leave with Arthur and the torture, the 130 fresh bruises, to come to the attention of TH's family, and doctors. She will not admit the salt poisoning because that intention was not formed in the spur of the moment, IMO. I don't think she dreamed for one minute that they would test him for salt, she thought she could convince medics and police he was into beating himself up and throwing himself into walls, because that's the story she tried on the hairdresser.
When police reviewed the evidence, spoke to witnesses, saw the reports to SS, saw the texts and the CCTV, got the experts to assess what had happened to Arthur, they knew this could and would not have happened without TH and ET working together. There was nothing separating their involvement in the torture of Arthur but the salt, and the salt did not kill him. TH was as much responsible because Arthur was at the point of dying, at the point of not being able to survive the next assault and that could have been and was until that point, coming from either and both of them.
TH was as responsible as ET. Say there were two people stabbing someone and one knife wound to the heart caused the death, that is joint enterprise murder. This isn't joint enterprise, I believe, because TH was out of the house. TH now denies most of his violent assaults,
"Hughes reiterates he never pressure pointed Arthur and states he gave 'little squeezes' to his neck as an affectionate gesture."
"[Prior QC] argues there is no logic in his stance not to plead guilty just so the jury can 'hear what I have to say before making a decision'."
"[Richmond QC] tells the jury Hughes is ultimately in 'your charge' and there was no real point in him standing up and pleading guilty."
and thinks he can get out of everything by saying he never meant what he said, and 'prove everything'. The point about that is that it shows no remorse and in the memory of Arthur he says 'I will lie about what I did to you and I will accept a not guilty verdict'.
Bottom line, if this was murder and they acted with the same level of culpability (bar the salt) he has to face the same charges as her, as secondary party because he was out for an hour.
When I look at the guidelines for "causing or allowing a child to suffer serious physical harm, or death", and the maximum sentence for it (14 years), this was a step up - intent to cause GBH. Intent being in their behaviour over the days before his death and in the escalation. Too egregious over its course to be classified as causing or allowing a death. If this had happened to an adult and not a child it would be classed as murder too. JMO
