She is not lying, which means saying things you know to be untrue.
Your mum is recounting false beliefs. She actually believes the things she says and it is very likely the cause is simply that she cannot remember - she says you're not visitting because she can't remember your going to see her, she says she didn't break the iron because she can't remember using it.
As her brain is affected by dementia, it will try to construct a story that explains things that do not make sense. The bedwetting for example.
She cannot remember doing it.
Her lifetime experience tells her that she doesn't wet the bed.
Therefore do not be surprised if you hear a story that someone else has done it. To everyone else a story that a stranger has wet her bed makes far less sense but to her it is the logical explanation.
The chocolate incident is likely caused by memory problems and loss of learnt inhibitions. Your mum fancied eating it, so she took it, ate it and then forgot.
There is no point in confronting her with evidence. She knows she is right and will reject evidence to the contrary. Or at best she will accept it, forget, and go back to her story.
Imagine you, yourself, know that you haven't eaten a bar of chocolate. You'd remember, right? And you've never take someone else's? Then someone comes along and says here, you've eaten my chocolate, here's the wrapper in your pocket.
The most likely outcome would be that you'd say they were lying and had planted the wrapper because you know you didn;t eat that candy. Or worse, you'd conclude they were trying to trick you and make you think you were going mad, or trying to blackmail you etc.
Same with your mum. It's pointless to confront her. Just deal with the practical problems. If the chocolate eating is becoming a problem liek making her sick, your only option is to make sure there are only small amounts she's got access to. You'll likely have to start doing the same thing with food, and particularly don't rely on her to not eat stuff that's out of date.