For the fans you definitely need the waterblock. Getting 25 fans running might work but we can not promise it since this equals to almost 160W what you need here to let them spin up. You should also keep in mind that the specifications from the manufacturers are wrong quite often so it is likely that they might even need much more power than that.

It might be a better solution to get some poweramps for the fans.

Right, each cathode must be connected to its inverter and the inverter to the multiswitch. Then you are able to control them trough the aquaero. Please not that when connected to the aquaero the multiswitch can only switch the channels on and off. To dim the cathodes the multiswitch must be connected to USB and controlled with the aquasuite software.

You can use the power connect adapter to switch the PC on and off but since the aquaero also connects to the PC as a regular HID keyboard it is also able to start (depends on BIOS settings) and turn off (depends on windows settings) the PC without additional accessories.

I don't understand your question about the temperature sensors. Every sensor that you connect to the aquaero can be used for monitoring and control.

Every flow sensor that we offer can be used with the aquaero 5 . Of course the high-flow sensor will work too.

If you want a secure shutdown method you will have to use the power connect adapter along with the 3-pin rely plug. The method with the HID keyboard would not work when your system had a crash. You can define a alarm shutdown event for every problem that you can imagine.

Yes, of course you can give every channel its own temperature regulation - also with the same sensor. So you could say that channel 3+4 only start at a temperature above 40°C while the other two channels have a completely different set of values.