"Despite all of this, at the end of the day, these SKs seem to be just built and wired completely differently"

I think it's THIS

I noticed from the get-go that my SKs33, 35 were very strange kids. I worked with youths in trouble for years already and I swear I never saw the likes of my SKs even in youths doing hard time. They were just very weird. I remember thinking that they needed some sort of corrective program and fantasized about giving them lengthy lockdowns like inmates get because nothing worked on them LOL.

Years later and even recently, DH told me that the reason he didn't parent them normally was because they were weird and didn't respond to anything like normal kids did. DH even confessed to me more than once that he noticed how "weird" SD35 was starting at age 2. It wasn't the divorce that made her weird. DH and BM were still happily married when SD was 2. DH said she had a deadpan face and didn't respond to playing like any other toddler in the family did. She would stand there with a cold deadpan stare with no emotion, no look on her face, just a stern look of judgement like you were crazy. DH admitted that SD was very cold throughout her childhood and he thinks that after the divorce that coldness is what scared the crap out of both parents to think that SD could just up and leave them one day without a second thought so both parents went nuts turning themselves inside out to try to capture SD's love and affection or any sign that she was just there. Now, SD has SGS4 and he looks right through you. Very strange. I don't think it's from being totally undisciplined. He's just a weird child who doesn't look anyone in the eye. He shows love for DH, but no one else. I can't get eye contact with him. I'm well aware that I have another weird child in my life after having to deal with SD and SS, but at least I can distance myself and not engage much since he's more on the periphery rather than shoved down my throat and in my home everyday.

I actually think SD caused DH's divorce. They were blissfully happy before SD turned two. I find it interesting that trouble began when SD started exhibiting signs of how "different" she was. 

Without saying a word, DD24 has told me she thinks there's "something wrong" with SGS because he's "different" and never looks anyone in the eye. He has tantrums over thin air that go on and on for at least an hour. He does it several times a day. He can't even be trusted to be taken out in public at this point. We've had some family events occur over the last year and it's been noticed that the entire clan no longer fawns all over SGS. The last two events, no one even acknowledged his presence. DH has two nieces who are quite blunt and have told me that people feel uncomfortable around SGS, so they stay back. He was cute as a baby but he's not very easy to connect with now. 

Selection bias maybe. The people who post here do so because they searched the internet in desperation to find someone who has experienced this. The people with well behaved skids, reasonable spouses with reasonable exes, supportive in-laws, etc., are out living their best lives. They never had to come here.

I dated a guy with 3 young kids about 15 years ago. Never once desperately searched the net to find an explanation for bizarre or unimaginable behaviors. We broke up for other reasons, but none that related to the garbage those on this site put up with. Those here have something unusually bad to deal with. 

1.  Selection bias, we are only here cuz we have probs

2.  SKs have been thru a family breakup, a traumatic experience for anyone.  But, their parents have over-compensated by indulging them and lowering boundaries 

3.  Parents have guilt complexes which prevent them from parenting normally

4.  Children are being P.ASsed so the other parent acts out of fear of losing contact

5.  Parents indulge the kids to gain favor, be the favorite parent.  A competition with the other parent

We ended up with partners that did not discipline their kids enough and the kids were allowed to get away with terrible behaviors. Then did not allow us as step parents to discipline their poor little kids either. 

We are mostly married to partners who are prior failed family breeders regardless of the details . We may also fall in that category ourselves.   We are in a community where people like us come to vent and find support.

Unfortunately while situations like ours are not the norm, neither are they particularly rare in the 2nd marriage/blended family universe.

There are a number of more recently established interesting divorce/blended family groups on Reddit, on FB, and other social media conduits.  Unlike not all that long ago where the SM /SParent groups were all love the Skids and you knew what you were getting into mass delusion groups.  We, SParents dealing with spineless SOs, their or our own toxic Xs, ill behaved spawn, toxic ILs, etc...,  had a less than warm welcome in those places.  Those communities are TBD regarding their quality, their longevity, or the relative quality of their members.  This is a very unique place/community and, dare I say family.

We have similar experiences because we are birds of a feather so to speak.

You mentioned examples of CODs who are well behaved successful adults with good heads on their shoulders.  You also mentioned boundaries. Then of course there are the spinless SOs with shit spawn and nasty Narc Xs.

Standards of behavior, standards of performance, and firmly enforced boundaries are the critical success factors in successful blended marriages and successful kid/SKid outcomes in those marriages. Just as they are critical success factors for successful intact initial marriages/families and joint non COD BK outcomes.  It is not rocket science. It is not easy, but it is not complicated.

It appears that another major commonality  among us is that generally setting and enforcing standards, boundaries, and adopting a zero tolerance position with the SO, the ILs and the StepSpawn  regarding anything but reasonable behaviors from them is a relative rarity.

Yes, I am a master of the obvious man-splaining savant.

They should stay together / good or bad/ until the kids are 18 yo.  Not take the easy way out.  The parents don't have responsibility of there actions. Don't care about there kids. Just care about themselves.   And do you really believe there story's.?

I admit I did when first told. BUT. they lived with this person and didn't know they were. 1. Nuts. 2 crazy. 3.  abusive 4 they screw around . 5. Didn't understand me  6.... 7.... 

'the SK are just following there parents. Self centered.  Self centered parents. Divorced and run gives you self center SK. 
'I just wonder if there any truly happy SP out there. 
 

P.S.   isn't it strange that the bio parent don't want a relationship with another divorced person with kids.  They want a un kid person to start up with.  They want to restart there life