Challenge is... Expose in the next few days or 2 weeks from now.
Prep your exposure materials and targets now, rather then when your emotions are scattered on upcoming D day. So that minutes after you talk to her. The emails and Facebook post or messages are going out. If the goal is for reconciliation... Expose to both your & her parents and closest friends who are friends of the marriage. Expose to their employer and OMs wife. Cheaters are liars... He could be playing your wife the sympathy angle. "My wife ignores my needs" which is a common tactic.
This will get bad really quick.
It's April and she already lied to you about Christmas party... So did they kiss or oral sex? Of course the OM and your WW can do a quickie in an empty room at work in 5-10mins. Enhances the thrill. Even non cheating co-workers have sex at work all the time.
A recommended evidence gathering way to do things when it comes to the cheaters phone. Don't just screenshot it and xfer it off the phone (this may leave a trail) but to use your phone to take photos of the conversations / evidence. Make sure some of the photos and or videos shows the cheaters phone itself. Yep dear, that's your phone with your case with my hand holding it in the photos.
When that day comes for you to have that serious talk. Have the kids out of the house. Have a VAR or your phone or other devices recording the conversation... This will not only give you more evidence, but gives you a recording of everything that was said. Because your emotions will be wonky, you may remember some things wrong or not at all or miss an important detail she said. Get a Sony VAR that has a USB port.
You don't tell divorce off the bat. But you threaten divorce if you don't get the full truth. That key phrase to ask is "...And what else...?". As long as you know she isn't telling you what you already know... Keep asking so she is spooked to tell you this you things you don't.
3+ months... Likelyhood of sexual intercourse is very high. What was the time frame between flattered to kiss? A week? A month? Pretty much any woman who gave me a BJ also shared the rest of her. They don't have to say that particular thing in texts since they also see, talk and have sex at work daily.
If she breaks down and admit all the info and you still wish to reconsiliation... You should have your requirements ready to go. Marriage counseling. Books on infidelity. She admits her actions to her own parents and siblings. (So she doesn't lie about why there are problems) , STD tests. Maybe a DNA test (for psychological? effect even if you know you're the dad) no contact. She transferres or quits her job.
You still expose to OM's wife.
If she doesn't want to stop the affair. Expose to her job and her family. Serve her papers.