Just in case you DO come back (and I hope you do).
I am really sorry that all of this has happened in the way it has. It sounds like a load of heartache all-round.
Fact is there are lots of "players" out there - the poly world isn't immune to them any more than the mono world.
I definitely agree that working on your marriage is what has to be done. But I think that she needs to work out whether this is something that may happen again - if she really is polyamorous, then it undoubtedly will.
There are some definite things that are positive about your relationship - the biggest one is that you knew about this, and it wasn't a secret affair. This means that there is a foundation of trust and open communication, which is absolutely HUGE, whether a marriage is monogamous or polyamorous.
If she really does feel that she is polyamorous, then, once the emotions of this phase have calmed down a bit, it's really important for the future of the marriage to have a talk about what happens when the next person comes along. Try to work out what worked for you and what aspects you would like to have differently - whether it's communications, boundaries or whatever. Hopefully this will decrease the odds of extreme drama in your future.
I really wish you luck.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb