I'm not sure what is causing it, but you are getting things like this
, InterpretationBox[Cell["", "KeyEvent"],
TextCell["", "KeyEvent"]]
in your notebooks. That is what is keeping Mathematica from seeing each of your cells as a simple numeric expression to turn into a single number.
I opened up your notebook using Notepad, looked for those, carefully deleted them, saved the notebook as Simplify2.nb (with type "all files" so that Notepad would not append a .txt onto the end of the file name. When editing that I remove the comma immediately before InterpretationBox and InterpretationBox and then CAREFULLY count opening and closing brackets to make certain I have all the arguments that are being given to it.
The edited simplify2.nb is attached.
I assume there is some combination of keystrokes and mouse clicks that is putting these in there when you are expecting something else. Perhaps someone better than I can explain can do this. But once they are in there they are completely invisible to the average user unless you know what to look for using Notepad or something similar.
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