I highly doubt you have the entire extent of things. You currently have no idea how deep this rabbit hole is. You may never know how far and you certainly will not have every single detail of it that you desperately need. Not a 2 year relationship. You need to start thinking whether you can accept that you will never have 100% of everything and whether you can live with that knowledge. Some people cannot and there's not a damn thing wrong with feeling like that. It is 100% valid. None of this is your fault.
Also, unfortunately, do not be surprised if the affair went underground and they found other means to communicate and to meet after you discovered the amount of texting between them. Did she have Facebook (in this day and age, who doesn't?) Instagram or Snapchat.. all those apps are easily concealable and do not show up on a phone bill. This is why you need to inform the OBS to see if she can discover anything from her end. Maybe you find an ally in your truth seeking, hopefully she is willing. But beware she may just want to stick her head in the sand and ignore it. Maybe this wasn't his first affair and she's learned to live with him. Sad but not unheard of, unfortunately.
Please follow through with the attorney visit. Filing for divorce is not going nuclear. Consider it like sending nuclear equipped bombers. Unlike missiles, you can always recall bombers. You can always end/dismiss the divorce up to the Judge signing the decree, and in reality, even after the decree is signed you can always move for a new trial or motion to vacate decree, so really, it's not the end.
On that note, you need to consider have some deep, honest conversations with your children. They are in their 20s so they are old enough to know the truth. You've seen the results of what 10 years of lies and deception lead to. Don't let your children and close family also stay in the dark and clueless. The truth -always- comes out; better it come out now and come from you rather than neighborhood gossip or another family member blurts something out years from now. Because of your WW's recent self-imposed mental crisis, it would be far too easy for you to be portrayed as a cold, uncaring and abusive spouse that leaves his wife after a metal health diagnosis. Maybe your WW facilitates this narrative, (she seems to embrace victimhood rather than face her flaws) or maybe, friends and family jump to this assumption because they don't know all the facts. You can't control what people think but you can inform those closest to you of the facts to let them come to their own conclusions. Try not to show them your anger (it will be difficult) but put it in a way that both you and your wife will need their support going forward.
You may want to express to your son that you understand if he needs counseling and to have someone to talk to. Offer to help him find a therapist. He may feel guilty and somehow this is his fault because it was his coach and he didn't (hopefully) see or detect anything between your WW and her AP. It's no more his fault than your fault and he needs to understand this.