I just quickly scanned over your post, as a side not I’m happy to read other people doing OpenCL stuff, I’m currently working on OpenCL stuff too and will start building JUCE applications but not to combine it with OpenCL but to use it for DSP.
Now to your questions, as I said, I did not do any work that connected OpenCL and OpenGL, but a single OpenGL context, suitable to render multiple Components was discussed here recently, mostly because of performance issues. You might want to have a read on this thread, im not sure but there might be a chance that it could help you 
I remember you having trouble back in the last year with that. I think it might be great if you would share how you did it