If you are having these kind of problems I would say definitely hold off on having a baby with him.  Upper 20s is old enough for them to have been out. I  think you them being out should have been a condition of you marrying him. I  know it's too late for that now. How did they end up living with you two instead of their mom anyway? Are they working? Maybe you could suggest they get a place together. Allow them to keep whatever money they earn until next summer to prepare them for their move, give them the deposit along with the first and second months rent (if the two of you can afford to),, and send them on their way.

he will only see (& not see) what he chooses.

Lifelong patterns are very hard to break, if not impossible.

You can try counseling, but honestly, there are very few who are truly equipped to handle our unique situations and marital statuses.

This may be your "only problem" but trust me, it's a HUGE PROBLEM. If he keeps finding ways to "patch things instead of fix them" and keep them under his roof and not show them the door, I'm afraid you're doomed. 

Well to be quite frank, it’s not your stepkids ruining your marriage, it’s your husband. 

"Any suggestions on how to help open my husband's eyes to their manipulation and to his refusal to make them move out?"

As they say, none are so blind as those who will not see. You can't force him to understand, particularly if, as is the case with so many of these fellows, he does not want to understand. All you can do is manage your own behavior. Make your boundaries and expectations clear to him and to yourself. That would include firm launch dates for the darlings. Rain hell on anyone who dares to violate your clear expectations.

Please do not have a baby until and unless these issues are completely resolved. This guy appears fairly clueless, so that could be never with him. It takes years for this type to come around, and honestly I think outside influence is minimal. They have to figure it out for themselves. You may have to look elsewhere for a good parenting partner.

I'm with the others, it's your husband ruining your marriage, not the kids. They should have long since been kicked out of the nest.  The fact that he refuses to believe what you tell him his "kids" do says he really shouldn't be in the "soulmate" category.  If I believed in soulmates, "not caring about my feelings" would not be high on the list of traits I'd look for.

The odds of you having a baby in your late 40s are slim, and it sounds as if you have no children - but please consider whether you want to be raising a young child in your 50s, especially if your marriage ends due to all of this nonsense.

agree, (as I have pointed out many times) the problem, as much as you want to blame his kids, is HIM. 

Another note to keep under your hat, the "first family" almost always dethrones the second one. Remember, they've had years of creating their dysfunctional dance together, which is their "normal." So, let's say you have the baby, do you see him magically transforming at the sight of this newborn person? I don't. If anything his "kids" will continue to guilt and manipulate him more, because after all, at this point you will have created a new heir to "their inheritance." 

That's whay  my DH's adult kids were worried about because I am younger than him. They were mortified I wanted a child with him. No way. I've already raised everything I'm going to raise. From now on it's dogs for me!

Good luck!

Late 20s is ridiculous for needing to live at Dad's unless there are extenuating circumstances ("I have cancer", not "I don't have a job"). I'd set a deadline and if they can't be out by then, for any reason, then you leave until they've got it all sorted out. I think a lot of step parents enable these problems. We didn't create them, but by tolerating them we keep the cycles going. You don't want to live with two late 20 somethings. period. If your husband doesn't choose his wife's happiness, I fear you chose the wrong man. And please, don't get pregnant with him until you're completely satisified with your lives.

Unles... they are absolutely respectful of their parent and the new spouse, and engaged positively in the new home and the new family.

Failed families are just that, failed families.  Any baggage from the failed family has no place in the new family. So... if an adult Skid chooses to be baggage, they need to GTF out. And in your case, Daddy needs to be the one providing his foot to their backsides to get them out.

That boot to the backside needs to be a burning platform that required the adult spawn to serve the home in a manner that is increasingly demanding on the adult spawn.  It worked for us when SS-28 graduated from HS and then turned 18 at the end of that summer.  We worked that kid's ass off.  He was our beck and call boy/chore bitch and cleaned, cooked, washed, etc.....  All day from the time we left for work until we returned home. His continued residence in our home was a day to day status based on his completion of his list of chores and duties.

Rather than giving the Skids a long horizon, like next summer, make their presence dependent on their daily behaviors.  Adult children are not permanent residents of a home or members of the family that life in that home.  They need clarity on this.  They are not there because their daddy allows it, they are there because you allow it. They and daddy need this clarity.

So... give it to them.