I need to confront W on this issue of the email account. I am concerned that she will see this as an ultimatum and decide to leave.
It's either a core boundary with you or it's not. It's HER choice as to whether or not her secrecy is more important to her than her marriage. That's the beauty of boundaries: if she balks, then you haven't really lost anything because tolerating infidelity, secrecy and disrespect in your marriage is not something you could have lived with anyway.
Doesn't make it EASY, but it really does make it SIMPLE.