Problems with mixing valve leak

[Rohan]

Here's a photo of the mixing valve in front of my water heater . Because of the angle and limited space, I can't really get a better photo.

Part A, the black elbow piece (seems to be an Aqualock/SeaTech 15mm Stackable elbow piece) has constantly been a problem. It feels like its in securely but then keeps popping out, dumping water into the bilge. But I have solved it for months at a time by jury rigging it, using string to hold it up and in place. I don't know what the problem is, it doesn't look worn, still looks new. But I have ordered a new part anyway, and maybe that will hold better. But I have an even worse problem now I'm writing about.

Part B is my problem, and a total mystery. I don't even know what to call this. It's some kind of metal piece designed to turn with a wrench, but has a plastic holder built into it for the black elbow piece below. So wishing to be thorough, I tried to screw that off with a wrench to see if I need to replace that also. It was stiff at first, but eventally turned. And turned. And turned. Now it just turns freely and won't come off no matter what I do. It won't go up and it won't go down. Yes, I have tried to spin it both ways. It just spins in place. And now it leaks there whenever the water pump is on, and I can't do anything to stop it!

Part C appears to be the fiber washer, and per this diagram, it looks like I can screw it off, and so MAYBE that will get part B off. But someone put sealant on it, which will make that much more difficult. I wanted to ask you all for advice first before I tried messing with that.

Another possible solution is to try and stop the leak with FlexGlue or plumber's putty. But I am worried about making it worse in the long run.

Any ideas other than calling a repairman?