Your new notebook still works fine for me. As for suggestions...
Don't use fancy characters as symbols in your code. If you end up writing a bunch of code that uses that character, it's tedious to type. Just typing a string of alpha characters is so much easier than inputting special characters. I wouldn't mention this at all if you were just using your overhat beta as a formal symbol, but you're assigning it a value, which means you want that symbol to be evaluated to some value every time it's encountered in your code, which suggests you'll be typing that overhat beta multiple times. That just gets so tedious.
You say you like to use "math notation" for clarity, but what I interpret that to mean is that you want what you display to other people to align with what they're familiar with. Clarity comes from understanding what your symbols mean, and you surely know what you intended your symbols to mean. I know I'm being presumptuous here, but I think you'd be better off using convenient symbols (convenient to type and with names you understand) for your code and then use Format or some other mechanism for creating what you will display to others.
I don't know the exact input mechanism you used, but the result is that your overhat beta definition is actually creating a DownValues value for OverHat. One consequence is that if anywhere in your code you assigned a value to plain beta, that would collide with your definition for overhat beta. Other than that, I don't think this should make a difference, but there might be some obscure reason why it's safer to create OwnValues for a symbol rather than DownValues for OverHat. You can use unicode to create an overhat beta that's just a symbol.
Lastly, and this is probably the least helpful, I'd suggest you use CompoundExpression rather than just newlines to separate "lines" of code. I have a vague memory that way back in the day newlines were interpreted as just white space, which then would often be interpreted as multiplication. So you really did need to use ; to separate expressions that you wanted evaluated independently. In recent times, I see lots of Mathematica code with expressions separated only by newlines instead of ;, but I just can't bring myself to break my old habits. I personally think not using ; it makes it very confusing to read notebooks, because you get all these output cells that you have to sort of reverse-collate back to the input code. I seriously doubt that this is causing your problem, but you might give it a try.
