I've been trying to figure out what you're describing. The 1-8 demultiplexer made no sense. A 5-bit ripple counter could work by counting the high res pulses each time the low res turns-on.

I assume once you have the count you'll latch it into a chip so it's held until the next count? You need to use a latch signal for this chip that transitions properly when the low res signal goes away.

I would have expected you to use the high res as the clock to the ripple counter. I suppose you could and the high res and low res so the clock pulses only occur when both exist. Definitely use the low res as the input. So, the counter begins counting up during the low res pulse and then when the low res pulse ends you latch the count into the latch chip.

I'm curious how you'll determine between the other 4 cylinders which have the equal 2 high res pulses. I suppose you could keep the count from the previous cylinder and then count the 2 pulse pattern on-top. Cylinder 4 would be 9 counts for example which is a unique count. This means you'd need a counter reset circuit that looked specifically for a 2-pulse pattern to reset the counter.

I'm not sure why you'd want to use the ignition module and then use it to drive another ignition transistor. If you use LS coils you drive them at logic levels since they have a built-in driver.

Best I can tell, the low res signal starts when the corresponding cylinder is at TDC. So, don't forget that you have to switch to the #8 cylinder when you've found the #1 TDC low res pulse so that the PCM can fire the #8 cylinder in advance of the #8 TDC low res signal appearing.