Is there any chance that she died 'accidentally' or early from him drugging her, and he panicked and had to bring the plan forwards? Or even that he wasn't planning to murder her, just wanted her confused and forgetful so he could get/use power of attorney?
I suppose there's always an infantismal chance it wasn't 'murder' in the strictest sense of the word. Or at least, not as premeditated as this appears.
However, if you have accidentally killed someone in ludicrous/suspicious/well dodgy circumstances, the first 📱 call you make should be to the cop shop so a post mortem will hopefully corroborate your unlikely story.
Not conceal her body in a sewage collection, kill the dog, concoct some cock and bull story about notes/shady henchmen, start making large withdrawals, try to dispose of assets, jet off to the sun and interrogate the neighbours about what they'd told the police.