Minecraft – How to /testforblock in the overworld from the nether

If you watch at about 2:50, you can see most of the left half of the circuit. The part for the stairs looks like it isn't shown very well but that is less important timing wise. Watch carefully over and over and see if you can replicate it this way. It looks like the repeaters for the middle activate and deactivate immediately. So the order (from a circuit standpoint) for both directions is M, L+R. This works for closing because the middle repeaters don't have anything to activate until the blocks are pushed into the middle. When opening, they will retract the middle immediately.

Breaking it down:

Opening

1. Middle repeaters deactivate (middle pistons retract)
2. Delay
3. Left and Right redstones deactivate (left and right pistons retract)

Closing

1. Middle repeaters activate (nothing happens because the middle pistons are not in position)
2. Delay
3. Left and Right redstones activate (left and right pistons extend)
4. Now the middle pistons have been pushed into position and are activated by the middle repeaters

So far the only way to prevent pigmen from spawning is to turn off your Nether portal. You can do that by dumping water on the inside of the portal frame using a bucket or dispenser with a full bucket inside. Lighting the portal doesn't stop the pigmen from spawning, because I have a very well lit portal and one still spawned nearby. According to this answer, zombie pigmen spawn inside the portal so you can't put a transparent block to prevent spawning either.