I think it was always his plan.
The fast amendment of the standing order shows me that.
The drugging and cold dispatch shows he had no emotional connection with Helen, from a time at least that they were making wedding plans and visiting venues. Even if Helen was planning on changing her will in the few days before her death, it doesn't explain 3 months of drugging.
I don't believe he thought he would use Helen's impaired state of mind to not kill her but only invoke the power of attorney. If Helen had to be assessed by a doctor they would run tests and all would be revealed. Heck even a stay at her mums for a brief recovery period would mean she would become drug-free and it would become clear this wasn't a permanent mental health issue.
So I do believe he waited until the will and poa were in place, didn't act immediately because that would stand out, then thought the time was perfect (she had signed the papers to sell the only property she had not bequeathed to him) to kill her, thinking the poa would permit him to spend her money, and get his hands on her estate when she was missing presumed dead, if he hadn't sold everything already by that stage with his false perception of a poa.
Seriously, I think this was long in the planning and other equally serious questions arise about his past. I think he will have been or is being investigated already, I don't think police will be waiting for the outcome of this trial.