The Ending: Are we to assume...(spoilers)
...that the maid goes to the gallows? That would be a suitably dramatic denouement, but I think it was badly set up.
The only witness to the alleged attempted murder/murder by her, the victim, is dead. If she tells the truth, she can cop to extortion (like William Hartnell) and there is no way in the world it can be proven that she administered the poison to Lowry. The circumstantial evidence is powerful, but the prosecution has to prove beyond a reasonable doubt that she did it, and I don't think they can. It would be more likely to a jury that the husband poisoned the wife (the maid had motive [a nice lifestyle] but so, presumably did Lowry - do we ever find out why he did it?) and there's certainly enough there to cast doubt on the prosecution case.
She would just have to say HE poisoned his wife, I blackmailed him, he wanted rid of me (see the unfortunate Mrs Burke) and this was the only way he could do it. It's unlikely, but, as I said, it throws enough doubt on the prosecution case to render it not proven "beyond a reasonable doubt".
Awight we're The Daamned we're a punk baand and this is called Carn't Be Appy T'day!