I don't think the way you have been living is sustainable, so anything different has got to be an improvement.  It remains to be seen whether your relationship with DH can survive, but for sure it wouldn't have survived if you had kept the arrangement you had before.  Your first obligation is to provide a safe environment for your 3 yr old, and somewhere where she is being physically attacked is not safe. Even if I didn't have a 3 yr old, I would have moved out, most probably.  Those twins sound like the spawn of Satan. 

So he has 6 kids total and you have 2? None together? If that is true, maintain your own place and he visits, great. If not, it wasn't meant to be. The problem is if the 3 yr old is his and he wants visitation without you present.  Sounds like you need to avoid the skids but if the relationship lasts they will always exist. You may be better off without them all including DH.

If I never saw my skids again, it wouldnt phase me one bit so I dont blame you.

to nibble the cone if you're deathly allergic to the ice cream?  To me it is akin to jamming a round peg into a square hole.  Would not recommend other than non serious recreational dating.  Personally I would move on and would rather be single than date a man with kids.

with the kids as young as yours and your DH, I think that you will find that the long distance relationship just won't work very well in the long term.  

I mean, you are looking at 12 plus years until his 6 year old twins would be out of the home.  That is an awfully long time.. and even then.. that won't fix the issue of him not being a good father and not parenting his kids effectively.  There are likely to be a lot of issues that will follow along with that.

Honestly, at this point, it sounds like he has zero business dating until he gets his home in order.

I my exH was "highly mobile" in the military the entire time we were married. 11 years together and there were some years we got to spend a TOTAL of 3-4 months together and other years were as little as 6-8 weeks. Never more than that. Our reason for divorce had nothing to do with our time apart, PTSD is just a bitch. I am not comparing the 2 situations as being the same thing but I just wanted to point out that lots of people have long-distance marriages and can make it work for a time.

If his kids are 6, you still have AT LEAST 12 years with them at home. That is a long time to be held hostage by some kids, which is essentially what is happening here. I think the reasoning behind some long distance marriages makes a difference too. I can't imagine that I wouldn't be incredibly resentful towards my husband and his kids if this was my choice in life: My bios hurt or living apart. 

I don't advocate for divorce or "run" on this site often, but my answer would be different if these girls were 14,15,16 too. A few years is one thing... over a decade is another entirely. I think it is probably best to either separate or he needs to scale back his custody schedule. Neither is an easy choice for him either. 

You really think the SD hitting 18 are going to leave.  You may be living apart for a really long time.   Part of being in in a marriage and relationship is to do thing together.  Living like this means that holidays will be separate, events will be separate. ( birthdays, holidays, school events, family things)  You are never going to vacation to together? 
 

I really don't think it's possible to do this for 12, 14 years or longer. 

To answer your question, NOPE.

I think the larger issue is if blending a family of TWELVE can ever work.

With violence toward your innocent toddler, it sounds like you have no choice.  It will be a long time until the 6 yo twins will launch, and by then you would lose your mind.  Living with twins would be a nightmare even if they weren't brats.

DH and I are recently separated but in our case it is only going to be for two and a half years before we reunite.  

You have to decide if he's worth it.  If he is, it will work, if not, it won't.