Gonna play devil's advocate here.
I agree with the first section of your post completely, she's a great character, smart, intelligent and someone who also helped the MC's career.
However, depending on choices made, there is a conversation between them outside the house where he asks her how much affection did you show me in the last 5 years? She doesn't answer this (rather tellingly). We also know (albeit from the MC himself) the cheating was a one-time thing. He hardly a serial womaniser with 'weaknesses'. So sure, he cheated, that's wrong, but the MC of the story (not us remember) believes he was justified...somewhat. So yes she is pissed and blames him...and rightly so...but she also understands she drove him to it. It seems to me, reading between the lines, that she very much went frigid after the birth.
Just nope. Jake never said that he thinks its justified. He excuses his cheating and gives reason for why it happened (like you explained with the lack of intimacy). And no she doesnt understand that she "drove" him to it, what kind of bullshit take is that?
And imo it was less about being frigid and more about both of them not makeing time for each other and priotising other parts of their lifes. And while Allison was confrontative about their issues (which leads to either solutions or an end of the relationship), Jake prefered to avoid/suppress their issues and instead did hide in the garage not even trying to tackle their problems due to conversation.
If you think your relationship lacks intimacy/sexlife you talk about the issue with your partner. If it doesnt work, you break up and look for women who give you what you need while still being able to have a normal(friendly) relationship with the mother of your child. These are solutions.
Cheating isnt a solution. Cheating only creates more problems. Cheating with a coworker of both yours and your wife is even worse, and the worst is when everyone in your company knows about it because your affair is a fucking golddigger or something like that.
Not saying either is right or wrong, but there are usually always reasons why someone cheats...it's not always because they are a piece of crap with no feelings. Because of this, the constant bitching and snide comments from her actually annoys me somewhat, even if they are understandable in context (and what usually happens in real life as people rarely if ever admit to their own shortcomings). Do I like her character now after this second release? Yes, very much so, is she completely blameless and everything falls on the MC? No...Should the MC have left instead of cheating? Sure...but that's not the story we have or what he did.
Nobody says Jake is a peace of crap with no feelings. Nothing is black and white and humans do stupid stuff from time to time. But its fairly easy to say cheating is wrong no matter the circumstances. Its ALWAYS better to break up instead if you cant work it out. There are no excuses for cheating and even less when youre cheating on a woman who gave you a child and obviosuly considers you as their soulmate.
Im completly sure with reversed roles (aka allison cheated on jake with a workmate because he didnt pounded her enough) you wouldnt consider it "somewhat" fair game of her to cheat with a workmate of jake.
And jake in this situation would be entirely entitled to being utterly pissed about her destroying their life together and the humiliation of all his workmates knowing about his wife being fucked by someone else in the company.
I even feel like shes too forgiving (if we look at it realsitic and not like a avn li, letting him sleep at her place and missing him after a betrayal like that).
In short: Cheating is never excusable. Jake was too weak to tackle his problems in a proper manner like either telling his wife what he wants and expects for their relationship or breaking up. Hiding in the garage instead of solving your problems and hoping they are just gonna vanish .. yeah no .. thats not how a man behaves.