I don't think she's being ridiculous at all.
I don't think chocolate every day is particularly appropriate for a 22mo.
I would be annoyed by a grandparent undermining healthy eating choices that you'd made for your child (we also have a 'naughty grandma' who thinks it's all jolly good fun to feed the baby cake we've said we don't want her to have processed sugar that young, and it's really fucking infuriating). Because our DD very rarely has processed sugar, when she does, she has a very obvious sugar rush and crash which is not much fun.
OP reading your second post about your mum's relationship with food, I can understand why your emotions are running high on this one. Your mum's mindset of treats and denials and charged relationship with food is a classic and unfortunately common one which as you've identified is a big risk factor for obesity. Chocolate advent calendars suit HER relationship with food, not yours and not the relationship you are trying hard to create for your child.
There's 2 issues here: practically, what to do about it. I'd suggest removing the chocolate tray and replacing some of the treats with e.g. raisins, dried cranberries. We do our advent calendar after breakfast as any sugar rush is least problematic at that time of the day!
Second issue: emotionally, how to deal with your anger at your mum sabotaging your food choices. This is a much more tricky long-term problem I think, but my approach would be making clear the boundaries that you are trying to create in your household wrt food, and requesting that your mum sticks by them.
I don't think you're being neurotic at all. A healthy relationship with food is a lifetime gift to your child. I'm regularly very very grateful to my mum for the attitude to food she handed down to me, which means I've never suffered from a destructive relationship with food and never had to watch what I eat to stay a healthy weight (yep I still would get as excited about sharing a pineapple as sharing chocolate ice-cream)
No, a little bit of chocolate won't kill you. But it's about the insidious attitudes towards food which are behind the gesture, and attitudes towards food can unfortunately have very long-term health consequences.