It will be from the PC rather than standalone
Also, is it possible to program it so that the linked LED changes each time a key is pressed? Not randomly, but in some form of cyclic sequence? For example, the first time I press "G", LED 1 lights up, the next time LED 5 lights up, then 22, then back to 1 etc.
Sure. It's software so anything is possible.The only question is who's going to write the software?
Will this be possible on the PC via the serial monitor though? As CrossRoads says, the program is only transmitted when the enter key is pressed, is there a way around this?