VB As I write, I'm trying to unpick your posts chronologically, asking questions as I go. I hope you'll be able to find them when you get a chance to respond.
I completely understand your need to cling to one another. When you get the book, you will read about your notions of safety being shattered. Shirley Glass treats infidelity using a post-traumatic recovery model and I think that's absolutely sound as an approach. Your H's safety has also been rocked though, because in all his thoughts on the run-up to this, I don't suppose he ever imagined that it would come to this point.
I can well imagine he thought that this would progress for a while and either burn out or develop. If the former, I expect he thought that you need never know. If the latter....well, he would cross that bridge when he came to it. I don't suppose he envisaged this rather different scenario.
Has he told you why he decided to come clean when he did?
Regarding love, I suspect you're right and he is putting the "in love" label onto his feelings, because that seems the most familiar to him. However, what I suspect he will learn is that he is in love with the feelings this new adventure offered - and not the person herself. A more accurate description might be infatuation, where she is concerned. Infatuation tends to be all about oneself - there is no great care or tenderness for the object of the infatuation, more a delicious sensation and buzz about the enterprise itself and an enjoyment of the mirroring, being told how wonderful one is, etc.
I'm interested that he spoke about marital unhappiness, because from what you're saying, that would be news to you. Was that a lie on his part - and his own mirroring of her tales of marital unhappiness? You may have seen other posts of mine where I chart this phenomenon, when the happier attached partner feels s/he has to mimic the relational unhappiness reported by the affair partner, even though it's not true.
Now for some other observations and questions.
When you get the book, you will read about taking a holistic approach to infidelity.
The first thing I want to point out is that your relational vulnerabilities are not unusual and wouldn't normally point to infidelity at all. Your marriage has been child-focused, but this is usual with a baby and young DCs and your sexual relationship sounds better tham most couples in your circumstances. There is clearly mutual respect and affection there. It's always a good idea after a catalyst like this, to review the relationship and see where it can be improved (for both of you) and arranging more couple time is sensible and desirable.
However....it is as clear as day that this isn't where the problem lies. Your H has got a huge set of personal vulnerabilities to infidelity. You describe him as "contained", that you are surprised at how much he has "thawed" and opened up. He sounds very low on the emotional honesty scale and the fact that he has no confidantes in his life, demonstrates this further.
For a man like this, creating a friendship with someone who seemed genuinely interested in what he had to say, giving him positive strokes by the bucketload, the effect must have been enormous. The irony is however, that this interest is rarely genuine, motivated as it is by sexual chemistry. Hence in the mirroring stage, new couples and friends look for areas of common interest, even feigning these along the way. It is an illusion, but contributes to the nonsense of being "soul mates".
There will be further individual vulnerabilities and to uncover them, you will need to spend time talking about your attitudes to fidelity - what you have individually felt was permissible in the past. Asking your H about why he thinks affairs happen and whether they are ever justifiable, will be interesting. Where were his personal boundaries, I wonder?
If he kissed a woman years ago and this survived undetected, then I feel sure he quickly allowed himself to get to this stage this time. He won't have needed the same permission-giving process as a person who had been completely faithful. What was he prepared to "allow" I wonder? What are his views about female sexuality? Does he pay equal attention to your sexual needs? What has he been giving to your relationship - how invested has he been?
Then there are the lifestyle vulnerabilities. Straight away, I noticed that he has a close friend who had an affair and left his wife for the OW and his only other friend knows both he and the OW. Did this person know what was going on and encourage it, I wonder? Is this workplace tolerant of infidelity? Is there travel away from home? Socialising overnight? What is the organisational discourse - the sort of conversations that occur about extra-marital sex?
I think there is also an elephant in the room about the OW. She needs to be humanised and discussed. At the moment she is a shadowy figure on the sidelines, albeit she is playing a very good game of pretending to be the understanding and benevolent friend of the marriage, by appearing to give him space. This is highly disingenuous and the agenda, transparent. I'd suggest you need to talk about her, her motivations, her aspirations for this relationship. You need to know far more about the conversations they have had and the intimacies shared.
I think it would be a good idea to discuss this with your PILs, as long as you can be sure they will be supportive and not in blame mode, of you, their son or more likely, the OW. If you need their support to babysit and give you time together, do ask for it. Their insights too, about your H's childhood and character traits might be illuminating. Many In-Laws in this situation report memories of their child not taking responsibility for mistakes, having a fear of confrontation, being unwilling to open up and share feelings etc.
You might want to reveal this to them in stages though. Perhaps, ask them to have the DCs for a couple of days next week so that you and your H can spend 48 hours together without interruption. It would be okay to say you've hit a bit of a rocky patch and need their help, without giving them details just yet.
What you need most of all as a couple right now is uninterrupted time.