You have already tried having difficult child come home to get ready for detox and rehab. more than a few times. You KNOW that until HE is ready, and not just yanking chains, it will do no good and just tell him he can continue to manipulate you and hurt the entire family.
It sounds like he is getting worried that he can't control you anymore and called the grandparents for reinforcements. This is a GOOD sign. A sign that his life is starting to get uncomfortable. he probably has quite a ways to go before he is willing to invest in rehab and really WANT to straighten out his life, but it is a small step toward it. So GOOD JOB!!
you will either have to ignore the grands or be up front and tell them how often you ahve done what they suggest only to find he stole things or had friends steal things and used drugs in your home and in your car and was a threat to all of you. THEY may need to go to al anon to understand this.
You can't control the grands anymore than you can control your difficult child. You can draw boundaries. Tell them that this is between you and your son and it is not open for discussion because your son lies to you and has done so over and over. They may, at some point, decide that they want him to come and stay with them until a bed opens up. You can discourage this, but you can't stop them. If they decide to do this, be SURE you tell them you will NOT be held responsible in ANY way for anything this 26yo man does. That if he steals, uses drugs in their home, hurts them because he is stoned, ends up with them losing their house because he is dealing drugs out of it (about the ONLY way to afford a drug habit is to sell drugs. Period.) and the govt can seize their home simply because he brought the drugs he is selling into the house with him, well, you can't fix those things for them. You will have to fight feeling bad because you KNOW he will do one or more of the above things. But you can't rescue them anymore than you can rescue your son.
Hopefully he will not take advantage of the grands this way, but you never know.
Stand strong! You are doing a great job!