Here are a couple of items that refer to the baking powder/baking soda issue:
http://www.koshercooking.com/resource/kosher.html
http://www.fatfree.com/archive/1997/nov/msg00312.html
And, I looked at a bunch of sources that contained the words sesame seeds and Passover. It seems that all of the sources that mention sesame seeds either say they are kitniot (although I don't know why) or they are listed as ingredients in Sephardic dishes. I called my mom, and she's as surprised as the rest of us were. I think she went off to do a search of her own.
As for finding Halvah that's labeled, it's most likely the same brand as the honey-sesame candies I enjoyed -- Joyva -- which is an Israeli product.
The things you learn every day!
Dina


