i have no idea on how to get the String object displayed onto the lcd
Can you display anything on the LCD? Or is that you can display some stuff, but not Strings?
You should do yourself a favor and get rid of all Strings from your code. Use NULL terminated char arrays, instead.
I see that most of your processing is done using a char array, and then the global char array is converted to a String object to be returned by the function. Those functions really don't need to return anything. Just use the global char array.