Hi Jules,
I think you missed a function call from your update to juce_Synthesiser.cpp, line 314.
I get
Error 1 error C2660: 'juce::SynthesiserVoice::stopNote' : function does not take 1 arguments c:\development\github\juce\modules\juce_audio_basics\synthesisers\juce_Synthesiser.cpp 314 1 ScopeSyncPlugin
Fixed by changing to:
voice->stopNote (0.0f, allowTailOff);
Thanks,
Will