It does sound like your SD has communication issues in general as well as social anxiety.

Did she have friends when she lived at her Mother's place?

My BS has social anxiety issues and I understand how difficult and frustrating it is getting through to them as all you want is to see them fully move into adulthood and have a full and interesting life. 

The key thing is to get your DH on board in getting her into therapy and from there getting her to launch. 

Sounds like your whole family system has communication issues. You can't tell DH point blank that you are fed up and need her to move out because you are afraid he will divorce you. DH can't tell SD what he really thinks (whatever that is) because he's afraid she will not speak to him again, so he texts her about it.  And SD can't say what she wants because she's afraid you guys will make her move out, or whatever her issue is.

I don't really understand why it's so hard for you to put your cards on the table with your DH. Do you really think he'll leave you and your two young children? I can't see that happening. Just make clear that she needs to be in the dorm by fall even if that means that you or DH does all the legwork to get her there. Period. Put him to the test.

My SD23 is the same - immature (emotional age is more like 12), social anxiety, can't communicate.  She expects everyone to intuit what she wants and gets upset if you get it wrong.  Communicate inappropriately via long texts, emails.  It is a huge struggle to get these kids to let go of the parental home as they are fear-stricken.

Your SD needs individual therapy to work on her anxiety.  Your SO and SD need joint therapy to build communication skills.  It is up to you if you want to joint that effort eventually.  In my case we did try family therapy to work on communication (SD23, SO and I) but it broke down and I am now disengaged.  But we did finally shove SD out of the nest.  She would not have left any other way.  Sometimes even adults need a push and SO needs to set that bar.

That said, it sounds like the uncertainty created by COVID is compounding SD's anxiety about living away from dad in a dorm.  This would be a great life lesson on life planning if SO were up to the task, i.e. well Plan A is that university will return to live classes in the fall and we'll plan for you to live in the dorm, Plan B is XXX...  We can't control future uncertainty but we can understand and think about what we'll do in certain situations to provide some comfort and relieve anxiety.

Meaning they should want to change.  It would probably be more productive to talk to SO about getting her an apartment with roommates near school if the dorms don't open.  That way you have some room to breathe and it is up to SD if / when she ever wants to grow up.

Momof2girls, your SD probably acts the way she does during these launching discussions when you are around because she probably knows you can see through her. Her intentions may be, regardless of how absurd, to live with you, her dad, and her younger siblings for as long as she can. 

I can't understand how these young adults don't feel out of place when looking at where their peers are. She also isn't setting a good example for her younger siblings. As a young woman she should be looking forward to graduating from college, getting her own job, and getting her own place she can decorate the way she likes. 

Btw is she on track to graduate college on time? How can she ever have a career if she doesn't even want to work on her issues to become a better person?  Your DH is a huge part of the problem. Just because one parent may have failed their child, doesn't mean the other one should too. Some of these parents make it seem so hard to make their adult kids launc into adulthood. It really isn't rocket science. 

Hundred percent agree with all this. 

The daughter won't move out without firm encouragement from the Dad, and it sounds as though he isn't really prepared to to do that.

It's a very difficult situation.

My two boys both left home at the end of high school and went to university. They spent the first year of uni in the halls of residence (dorms) and made friends there. They went flatting the following year with some of those friends. 

That's how kids grow, and make friends, develop networks. 

None of those things will occur if kids live at home forever, sheltered and cosseted by Mummy and Daddy...

My exes daughter wife signed up for uni but pulled out before classes began. She refused to go into the halls, said she was going to live at home forever.

This was about 5 years ago, and yes she's still there with no signs of moving on.