The spirit element binds the circle together so everyones powers are heightened and more powerful.
Spirit witches are able to use any element, this makes Ariella the strongest, however Jane isn't trained so don't worry about me god-modding my own character. xD It would take a lot of training to get to Ariella's standard.
They can also have minor healing abilities, so can heal small wounds.
They can communicate with the "good" spirits. This communication makes them more prone to evil power as if they consort with "evil" spirits, the may draw them over to the evil side by promising them infinate power.
So yes! Your spirit witch~
If a witch was turned, they would lose their mental capabilities, driving them insane. They would also be cut off from nature, so they wouldn't be able to use their powers anymore. Essentially, they would just be a vampire and no longer a witch. Depending on the strength of the witch, would depend on whether or not they would die from the stress of the change.