How much foreign substance can I add to processed cheese? Recently, I decided to make myself a soup with a new kind of cheese nockerln. For the nockerln, I wanted to make nut-mushroom cheese. 

I used roughly equal amounts of emmentaler, portobellos (pureed) and hazelnut flour, with some heavy cream, some gluten and a pinch of lecithin. The mixture turned out very soft and very gritty, even after prolonged refrigeration. The taste was good, but the texture was all wrong. I was aware that the end result would be softer than pure emmentaler, but I wanted it to be something which can be picked up without smearing (harder than the usual wedges of processed cheese). 

Is there a good guideline/chart for what amount of proteins, solids, fat and water can I add to cheese to get a certain firmness of the final product?
I would experiment with more cheese and less added liquid to make it more firm, making sure the pureed mushrooms are as tiny as possible (Pacotize, homogenize, etc.), and if that still gives an inferior texture try adding some sodium citrate. It sounds like you want to add a minimal amount of liquid and enough emulsification to make sure the water-soluble parts of the mushrooms and flavoring stay put.