I have this same exact situation right now with my 6 year old BD Leila. I came on to maybe find some infromation and I came across this post, the way you described your beardie is exactly what mine is like. Very light, dark beard, mouth open a little, eyes half closed, not very responsive to anything, but she's def still alive in there (she shows signs of movement when I hold her and in her throat). I'm waiting to see what happens over night. She was perfectly fine yesterday, and I noticed this when I woke up today. It didn't seem to get much better as the day went on. SHe let me turn her all the way on her back which she NEVRE EVER allows, that was the red flag for me. I know they are odd creatures and I've been though enough burmations with her, so I know they can pretty much shut down but this seems a little different to me.
First thing I did was check lighting, temps, and humidity. Her viv has been the same and perfect for her for years now and I monitor it very closely. I'm so sad to hear yours passed. I have a feeling mine may be on her last leg and it's killing me inside to the core. I scrolled though here to see if anyone had advice or had the same situation and could shed some light on it. The only thing I can do is see what happens over night. =(