Correct me if I'm wrong anywher here .

1. When using optional extension cable it will send the shutdown signal to the motherboard by simulating holding down the power button of the case.

2. Yes, the temperature probes are the same for Aquaero 4 and 5.

3. It should be, I've been using similar (not transpatent) with Aquaero 4, so I should be no problem.

4. It is actually using a regulated volatage source. And since speed is increased almost linear with voltage it's quite easy to control it this way. The temperature of the coolant will be actually near the same all the time - the difference between "hot" and "cold' liquid for the radiator is near 1,5 K (it depends on the radiator and fans though). It's better to use the temperature of the components. Eg. the hottest parts in my case are the CPU and two GPUs. So each of them have a temperature sensor attached. Note, that you are monitoring the temperatures of water block not CPU/GPU directly this way. So eg. when my CPU is IDLE it has 43 C while the water block of it has 38 C, while under full load it's 47 and 42 C. So, the difference is constant and equals to 5. But when it comes to graphic cards when IDLE I'm getting 45 and 43,5 C and while under load it's 55 and 45,7. So in this case it's not linear - the part of water block with the sensor is made of plastic, so it gets saturated easily. Under higher temperatues it's much easier for it to give away heat to coolant than air. Ok, so under heavy load the average of this three water blocks temps is 45. But this is when using full speeds of radiator fans. Since, we don't want to keep them this loud, let's turn them off. Now, the temperatures are rising fast. I stopped at 50 for CPU and 60 for GPUs. When recalculated to water blocks this gives an average of 48. So my maximum temperature shouldn't be larger. Since Aquaero has a nice built-in PID controler (read a little on control theory and it will help you) you can use 48 C as the target value and the radiator fans as an actor. So, after powering off and letting things cool a bit I started the system the radiator fans were off for a few minutes, and while the average temperature got to 48,3 (read later while 0,3 more than set) the fans went on, they speed up a little every a while and finally the temperature stoped rising on 49, went down to 48 and the fans stopped. The 0,3 error margin (hysteresis) is there to prevent getting them on and off every few seconds while the temperature is jumping between eg. 47,9 and 48. To make it work better I increased the power factor a bit and decreased the integration time, next time it only went to 48,5 and not 49 so the over-regulation was smaller as expected. Finally, the result is having 50 C for CPU and 60 C for GPUs under full load. While in IDLE the themperatures are lower and ocasionally the fans are fully off for some time. Finding the right setting is quite fun, unless you're working with PID regulators everyday as me .

1.) When you use the ATX adapter cable you cut the green power on cable and connect it to the relay. As long as the connection (in the relay) exists, the PSU will run. In case of an alarm the relay will switch to the other state, the connection get interrupted and the PSU turns off immediately.

The method mentioned by aioneru uses a parallel connection to the power button of your case. It will "push" and hold the button until the PC turns off.