It's not likely you'll overdose on nicotine just from smoking cigarettes. Your body absorbs the nicotine in a cigarette around 1 milligram when you smoke it. Overdosing from nicotine gum or a patch is rare but it's possible if you don't follow the instructions carefully.
I would say parentheses but if you are on about a thing before I would say brakets
I believe that the phrase from Mandela's speech that is an example of a metaphor is the first one - 'the children, [...] the greatest of our treasures."
A metaphor is a type of comparison, where two or more things are compared without using words such as <em>like </em>or <em>as. </em>Here, the children are the greatest of our treasures - they are compared to a treasure, something which is the most important thing of all.