Generating a short pulse after a delay on ATmega 328/P
I didn't read the codes that closely but it shouldn't this complex. Assuming the desired duration is reasonable large vs. Isr overhead, all you need is to use the signal to trigger an Isr (a timer cannot be set up as an external interrupt) and in that Isr ttriigger a series of delayed pin actions.

The situation gets tricky if the desired duration is very short.