First, Alie, my deepest sympathies on the loss of your DD. My heart goes out to you! Your grief is still probably quite fresh, I imagine, and it must be awful to have to deal with all these other issues on top of it. I am so very sorry.
I feel bad for K, as well, and I can see you do, also. It's beautiful, IMO, that you are so supportive of him. And wise, too, since, right now, he is your main connection with your younger GC.
You love your older GSs, too, though, and I imagine you value your relationship with them, as well. So while you "hate" what 2 of them "have done" - I think you mean turning on K? - I hope you can manage not to get in the middle/take sides.
That's why I'm concerned about what you want to say to M. If you're thinking of defending K, please don't. Too much chance of it's backfiring on you. They're adults, they need to work out their issues with each other or through the courts if it comes to that. True, K may not be up to speaking for himself, right now (sigh), but, IMO, you can't do it for him.
If you think he has more information about the 14-year-old and want to ask about that, fine. But I'm getting the impression that M was working along with the half brother who threatened K. Have I got that right? If so, and you want to move the boy out of the half brother's home, then discussing it with M is probably not such a good idea, IMO. It's only likely to end in a bitter argument. Better to work through the SW. I just hope she gets together with you soon.
But I'm confused - isn't the half brother the 14's half brother (or full brother), too? If so, why are you worried about the boy's living with him. Is it cuz he threatened K? Is K the boy's dad or a close stepdad? In either of those circumstances, I can see not wanting the boy to be with someone who is so against K. But unless you have other reasons to be concerned about his being there, for now, please take heart in the idea that he is safe and cared for.
Beyond that, it's a tricky situation, IMO. I doubt M would have more to say about it than to express his own biases. Again, I think you are better off waiting to talk to the SW. (If she cancels again, you can go to her supervisor.)
Still, I figure you should reply to M. It's the polite thing to do, his text was loving, and not answering might be taken as a sign that you're against him. But getting into a conversation... well, I don't know,,, it could end badly. Maybe just a brief text and see how it goes from there?
I hope and pray that everything works out for the best.