Stop telling them to bury the hatchet. Act as if it has absolutely nothing to do with you. And in reality it really doesn't anymore. Let her handle her crap. Plus you have enough shite to deal with. Why add more? There is absolutely no benefit to YOU to discuss such matters. In fact it could be detrimental for YOU.Why? Because you're dragged into something that could be seen as you siding with one or the other. W could view it as ganging up on her. But more importantly YOU don't like being involved.
This is hard, but it is fundamental that you organise your life to suit YOU.RRemove negativity and anything you don't want. You and you alone get to choose what is in your life. A simplistic view, but try working towards it. Looking at it that way, you have the opportunity to builda most fulfilling life. It is an opportunity.
You may not see that now, but it is true.
I am sorry for the speed at which you got served. It changes nothing. It only confirms that she is unhappy and wants out. In your shoes I would get an attorney and I would tell her so, though I would probably wait until after meeting him/her. Knowledge is power.
Do not block the divorce proceedings. Accept it. But let her do all the work. Stall for time when you can. Don't rush your decisions.Be sure of each step as it could affect the rest of your life. Validate her desire to divorce. State it is not what you wanted (I use the past tense on purpose..... she does not need to know your current thinking IMO) but you will not fight it.
As for OM, I won't tell you what you should do, as I am not sure what I would do in your place. I think I would bring it up with her in person. And say that you have heard rumours and would appreciate knowing the truth. Your kids may hear these rumours too, so knowing will help you reply to them.
Just be prepared. She may lie. If she admits it, do not show your anger. Maybethis would help you back out of that friend zone. Be prepared that it could be true. I will share some words that are easy to write but much harder to live. Accept it as being the truth and seeing as ye are on the path to divorce it is now out of your business, except in relation to your kids. Protect them . Other than that she is free to live her life. If that is her path you will be better off with minimal contact.
This is not easy. Be strong. Best wishes
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together