I haven't unlocked mining depths, but I got Sherry Ex2 from the method used above. I would recommend Piers or Sherry (or if possible, Ada) just because they are the ones most likely to be unavailable to you when you start playing mercs/extra content.
Also, does anyone have the link to Ada's Ex2? I can't seem to get to it on the site.
Edit: Seeing as how I dont have Jake's campaign beat, and seeing as how that was part of the original way to unlock Ada, does that mean I have to beat Jake's (along with Chris and Leon) first?
I still think Pac-Man was the best game and had the best storyline... EVER!!! 'nuff said.