OK so I have been able to solve my problems. I have a class acting like the Abstract FIFO / stack where I have an atomic pointer for the write position. At read, I look for this write position, and start reading at that position - N samples.
I have realized also that a few functions / classes in JUCE have been made a long time ago because C++11 has been slow to come up, such as the JUCE atomic class, and since I’m using now C++11 everywhere I should use for example std::atomic instead 
I do use also the AsyncUpdater class to call the repaint, and at choice a thread or the threaded HighResolutionTimer class to trigger the AsyncUpdater, instead of a Timer.