"1. What are the 'L' utility?"

This is standard 'C' notation: it tells the compiler to treat the number as a long - presumably to avoid overflow/truncation problems?
See K&R or any other good 'C' book for further details.

UART_Putchar("A");

if you want to put a single character, that should be

UART_Putchar('A');

Again, see your 'C' book for further details