For point two, yes. Put Snape in the additional likes and dislikes section, because the only characters who should be in the sections above are those that show up on the official table (remember, this exchange focuses on the characters that people usually don't like). I will definitely assign your request to somebody who enjoys writing Snape, so you'll probably get a Snape story anyway.
For point one: not all the Weasley family is included in list C. What you might want to do is the following: have your "characters to exclude" list look like this:
Molly Arthur (both of whom are on the list above) Bill Ginny Ron Lucius Narcissa Draco (all six of whom are on list C) The Others
That makes nine official names to exclude. You can add one more, either from category C or from category A (i.e., the characters who are the focus of this exchange).