I wouldn't be surprised if Mindy leaked the story to the press herself as she is probably the most cold-blooded, attention-seeking, self-promoting person I've ever met. She may be an emotionally-damaged person as you labeled her in your post but she's also manipulative and prettly much always looking out for number one.
Mindy's dad, Tim, is raising her son in FL. Mindy hasn't seen the kid in at least a year yet a couple months ago she was on "Extra" pimping herself out talking about how her son is the most important thing in her life.
The only thing Mindy cares about is self-promotion and if she's lucky, getting back on the County Fair concert circuit.
On the other hand, it makes sense, if this e-mailer's words are to be believed. I mean, it's horrible to think that someone would do something like this, leak news of her own affair to either the press or the lawyers for Brian McNamee, who have probably been telling anyone who would listen that they're looking for dirt on Clemens. When you consider that their supposed 10-year affair has presumably been over for several years, McCready isn't likely to burn any bridges with the Rocket by making this public anyway.
If you want to know who leaked this information, one way of going about it is to take Deep Throat's advice to Woodward and Bernstein and "follow the money." In this case, though, we'd have to follow the potentialmoney, and the only one who stood to make any money off of this information was Mindy McCready. Sure, the McNamee camp stood to gain from it, but how would they even know it was out there if someone didn't leak the information to them? With a new CD, a TV show and a documentary coming out all about the same time, but her fame as a country singer long-since past, how else could she get herself back in the public eye?
Another Update: The NY Daily News is reporting that their source has indicated other affairs that Clemens had, and going so far as to actually name one of them. Angela Moyer, a former bartender in Manhattan during Clemens' tenure in Yankee Pinstripes. They also mention women in Boston and even California. I guess he needed someone for those long West Coast trips. At least this one was legal at the time. She's evidently 30, which means that if she was involved with Clemens when he joined the Yankees, she'd have been 22, while he was 36. And married.