Re: Ideas for a "snake-themed" half villainess
I'd add onto the "Desolidification, Cannot Pass Thru Solid Barriers", "Does Not Protect Against Damage" (-1) and "Requires a Contortionist Roll" (-1/2) and of course, the actual Contortionist skill. I'd blow 5 - 7 points on it as well, to make sure she's not going to blow the roll.
Maybe you could also give her Ophidomancy, ie: divining the future through the movement of snakes (even it's only to make the PCs think she is a seeress).
An ability to Summon snakes would be appropriate, as well.
Hope this helps and good luck.