I was wandering if someone could please help me find some answers to some questions I have re matcha tea?

1) Is one cup of matcha the equivalent to a cup of coffee in terms of how much caffeine it has?

2) The packet of matcha I bought says to just put half a teaspoon instead of a whole one could there be a reason to why this is?

3) I have read that matcha can be harmful as it can contain lead and also affect your liver due to the high amount of antioxidants in it - how many cups are too much? Also should I be worried about it harming me?