He will definitely get credit for the six months he did in county. He probably has to do 85% of the sentence (assuming he stays out of trouble and remains incident-report free). That would be 25.5 months. What jurisdiction did he get sentenced in? Where did you see the release date (that might not have figured in the good time as of that listing)? 

Missing the court date is not a good thing but if he was already in jail on another warrant, it shouldn't affect him for missing. BUT, there is always more to the story and if his court dates about domestic violence what was the warrant for?  Because of the limited information, we have no idea how long he'll have to sit.

We need to know more about the sentence that was imposed and what his release conditions are, probation period, etc before we could give an answer relating to 2020. Good time is only 15% unless he has a deal with the prosecutor to get more time off for cooperation

Once the offender has done their required time, they are held in the A & O area until the state with the warrant comes to pick them up. There are usually no delays as they state with the detainer knows the inmate's release date. They are oftentimes waiting in the parking lot for the actual release to be finalized.

Yes, they refer to this as a "detainer".

If they are scheduled for release on the 28th, what is the court date about? If another county is picking him up, then he has a "detainer" on him. That is when another jurisdiction requests a hold on that person so they can bring them before the judge in their county that issued the warrant.

The short answer is "yes". A three year sentence (36 months) with 15% good time drops it to 31 months. 

Graduating from the drug treatment program is also granting immediate release. They oftentimes take the souse when there are openings in the program and don't always fit release dates. We would advise you call the facility and speak to his counselor... they will; know the out date for sure.

Could be... if Virginia only wants him for 31 days, then that is possible

Your husband should know for sure. If you can't communicate with him, call the counselor at the facility, if the sentence calculation has been finalized, there will be an "out-date" which is when your inmate will be released and this person will surely have that information.