I think it is more likely the so-called friend (what-was-her-name-now?) dropped in on Vera and gave her a hand with the pastry making. I cannot see Vera putting poison in pastry in order to eat it herself . Why bother baking? Just throw it in some whisky and knock it back or even tea for that matter. If you are going to kill yourself, why go to the fuss of baking a pie? Vestiges in the glass would incriminate as much as pie left-overs. Or even: the unpleasant friend put poison in the pastry to kill Bates because Vera was so frightened of him and she thought she'd do her a favour by bumping him off but she forgot to tell Vera...
I didn't pick up at all on Bates being Irish. How do you all know he is supposed to be Irish?