When I initially looked at your attachment, I immediately thought of Computer Braille Code. Upon further thought, I agree you would not use the calculator key symbol, and that you could continue in Nemeth Code. I would not use contractions for the items in the 2nd coloumn. It is my opinion that it would be obvious to the reader (from the context) what is meant by CTRL and follow the same format as the other entries with the plus sign unspaced. I'm sorry that I cannot point you to a specific rule that confirms this.
If these are used often throughout a chapter, at the first occurance you could insert a transcriber's note explaining how you are handling them in braille.