Milhouse:
That’s interesting about Sanz; I was not aware of that.
It’s not entirely true that all say shehakol on raw potatoes. The Mishnah Berurah says no brochah is made on them sense they aren’t really eaten like that if I recall correctly.
I believe there is a shittah independent of the Sanz minhag you mentioned that karpas should have to be naturally occurring adamah, not something that has to be cooked into it. I’ll try to track down a source if I can.
Regarding onions, isn’t the Minhag haGra and minhag Lubavitch to use onions for karpas?