I was really sorry to read your story. Man, you're off earning a living (not to mention defending our freedom), and your wife is back here misbehaving and sending you -- literally -- a "Dear John" letter. My first thought was "what a b*tch."
(OK, actually that was my SECOND thought, too).
I'd recommend the keylogger, not to know what she's THINKING as much as to know what she's DOING. I'd be highly, HIGHLY surprised if she's not already deeply involved with someone, as she seems to be displaying all of the classic signs. And if this is infidelity, it calls for a slightly different (but overlapping) set of techniques.
I would not advise ML to her, but if you do, please use protection.
I know this sukks. Life has handed you a great big you-know-what sandwich, but it is what it is. All you can do now is fight thru it and work on YOU, and we can help you with that.
