I am so sorry you are having such a struggle with your MIL! Kudos to you for sticking to what you know is right for your little one and for the strength it takes to constantly battle opposing (and incorrect!) views. Here is some more info you might want to share with your MIL:
Does my baby need vitamins?
The Truth About Iron: Do Breastfed Babies Need Supplements?
Is iron supplementation necessary?
Index - Vitamins for Mom & Baby
The LLL meeting sounds like a great idea! Weill your MIL feel comfortable asking the leader questions? That would be wonderful for her to hear somebody else support everything you've been telling her. Good luck!