How do I make sugarfree chocolate mousse made with 100% cacao chocolate less stiff?
So there are a few things you could change here. 

Firstly I would use 85g of dark chocolate for every 2 egg whites. 
When melting your chocolate, 2 tablespoons of hot water will help smooth it out stopping that graininess and make it less stiff/easier for the egg white stage. 
Leave to cool and whip up your egg whites. Then fold in a third of the egg whites, then fold in the rest. 

If you don't want to use cream, I would recommend trying 50g of greek yoghurt into the cooled chocolate before adding the egg whites. This is not as heavy as cream, will keep the smoothness of the chocolate and also keep it light and fluffy.