I realize now that I certainly didn't answer your question concerning the difference between "mêler de" and "mêler à".
The quotation from the Robert (thanks Nywoe) gives a good example:
- In the phrase "mêler des détails à un récit", le récit is the main 'ingredient', while les détails are the 'salt and pepper'.
- In the phrase 'mêler un pamphlet d'allusions médisantes", le pamphlet is main 'ingredient', while les allusions are the 'salt and pepper'
I don't know if my culinary metaphor makes it any clearer, but I hope so
