Hello back to you, Devastated Mom. I'm glad that you are here.
My story does differ from yours but in reading your posts I see you really are between a rock and a hard place. I am so, so sorry this is happening to you and your family.
Your D18 does not have a good track record, for sure. There doesn't seem to be any indication that she has recently received/accepted help for her anger issues and there are certainly no guarantees that she will not physically abuse someone again when/if she returns to your home. That is definitely not a position you should have to put your household into again. Meanwhile, your D16 deserves and is entitled to be safe in her home.
I think it is good that the therapist who is working with D16 has also worked with D18. There is not a lot of explaining of family history/dynamics that has to happen with her/him.
You wrote... ."I realize I am the only one who can make this decision... ." and that, sadly, is the same for all of us. We arm ourselves with as much information as we can and then hope/pray that the decision we make will be the one that results in positive changes. Whatever, Devastated-Mom, be confident in knowing that you are doing the best you can possibly do.
As fogged as D18's brain may be, she too is capable of making choices. There is probably a "honeymoon-period" when she first comes into her grandmother's house... .and maybe the same when she has returned to yours... .a period of time when she follows the rules of the house. That shows that she is capable of making choices.
All I can add is that I hope you stick with us... .keep pouring out your heart

... .keep releasing some of the steam to an audience that is listening and understanding... .and will continue to give you as much support as possible as you move forward.
Huat