Say "You know that I cannot eat this, so why do you keep getting it for me?"
Then, throw the chocolate away. Right in front of her.
Well, it sounds like the OP did the next best thing, by giving it to her FIL. At which MIL got very angry - why, I wonder? She says she *knows* the OP cannot eat it. Her argument that she has to make her presents "all the same" holds very little water, when that means something like "5 people are happy, and the 6th is left staring at something that is, to her, rank poison."
There's something very passive-aggressive going on her, and I'm not sure what. This is a power struggle, that "OP will TAKE the chocolate, and she will LIKE it!" I'd love to know what she really meant by "it didn't go how she'd expected." The OP would have taken a bite, and discovered that she really *could* eat chocolate, if she just tried it?
Anyway, cheers for DH in standing up to MIL for it.
OR, save all of the chocolate that she gives you, and re-gift it right back to her on the next gift-giving occasion.
Honestly? I think this is the best option. Maybe it will sink in when it all comes back to her.