It won't make the slightest difference whether you use '1' '2' '3' etc or 'F' 'B' 'L' etc. Just use whichever you prefer - personally I like the letters because they are informative.
You seem to have no code in your program to tell you what characters are actually being received.
Make a backup copy of your program and then delete all the directions except FWD and STOP and concentrate on getting those working reliably.
...R