Don't call the cupcake bakery. Don't try to find another guest who attended the wedding. Don't call the venue again. This is taking too much time and causing too much angst for you.
If she calls again (and I agree with the other posts about not accepting abusive language from her), calmly tell her that according to your records and the research you have done, it is not even possible for her to have had raspberry filling in her wedding cake. Tell her if she has proof in the form of leftover cake or a photo that clearly proves the wrong type of filling, then you will consider talking about the possibility of a discount on her cake (NOT a refund). The burden of proof is on her.
By the way, beware of how people can alter photos in Photoshop!
"Dessert is probably the most important stage of the meal, since it will be the last thing your guests remember before they pass out all over the table."
The Anarchist Cookbook
"Dessert is probably the most important stage of the meal, since it will be the last thing your guests remember before they pass out all over the table."
The Anarchist Cookbook