Consciousness is perhaps the most glaring one (as far as current models offered by natural sciences are concerned), if nothing else then because we deal with it every moment of existence or because it is what natural science itself stems from, is shaped by, is dependent on and exists because of.
Of course, whether it is the only unknown is unknown.
No, but I’m familiar with his take on consciousness and it is more or less what I mean by thinking in the box of existing models and trying to explain away something inconvenient to them.
Of course, whether it is the only unknown is unknown.