Newbie trying to fix old TV/Monitor

@jeronbaffom little things like this "- The standby LED (red) and the power LED (green) seems to be both working fine" is really non descriptive for a proper troubleshooting. What is important to know is if they do blink in a pattern. Most TV's use the power LED blink code for an indication that the TV has some sort of board issue and is not passing it's POST (Power On Self Test). For example on you TV a 5 blink code from the standby LED means there is an issues with the T-con board.

Since yours is not blinking, your TV does not register a board error. Now you say that you "Even pointing a flashlight on the screen, I still could not find any active pixel" that would mean that no image is being generated. With a failed backlight you will always make out an image, even if the backlight error comes from the power board vs. the backlight strips.

Since you have neither audio nor video, a T-con board would be very odd. The audio is being generated on the main board as well as the video. If you would have audio but no video I would suggest to check the T-con board but since you have neither, I would skip that for now.

This "The power board seems to be properly supplying 12V to the main board" needs to be better explained. We always need to know where you measure and exactly what you have measured. For that you want to check the power board to main board connector. There should be a legend that indicated what voltage you find on what pin. Check all the voltages on CN6402 and 6403 as well

Adding images to an existing question

For example Pin 3 is the power on connector. It should not show 12V. It could show 3V but when you turn your TV on it should go high then possible decrease or stay high. Check to make sure that this is happening. Consider this. It is your IR board that tells the main board, to signal the power board, to switch from standby voltage to supply fully voltages to all circuit. Now, remember since we can't see what you see, you want to go ahead and post some well focused pictures of your boards and those connectors as well. That will help to narrow things down.

Always check the ribbon cables. Disconnect them, check the contacts for corrosion as well as tears and rips. Make sure that each cable is properly seated. As for "How difficult should this repair be?" could be simply a corroded pin or a failed fuse on the power board. Maybe the panel does not get any power because of a wiring issue. There is never an easy answer to that sine worst case scenario it is a bad panel, now it is a catastrophic failure :-)

Again, don't be discouraged. Use it to go through the motion and learn the basic. I simplified a lot since schematics are very difficult to get for most of the boards. That is why it is very important to identify the boards since they may have been used on different TV's as well. The more specific information you can gather, the further in depth you can go, and really get a good handle on it. Guess what? Sometimes we can just never figure it out and we'll have to take a step back and put the device aside until more information is available. We all have our limits and it is always good to know those limits. Absolutely no shame in that.

Now with all this said. One word of wisdom from my own experience. Watch those power supplies! They pack a punch. You do have household current on that and it can "bite" you pretty hard. Always disconnect your main power and always discharge the capacitors. Never touch anything directly on the power board until you have done that. Caps can hold power for quite some time....it hurts :-)

Make sure pets and small kids etc. are being kept safe as well.

You ready. You have the service manual but you did not link to the electrical manual that has the schematic for the power board. That is KDL 32EX405 Schematic this one. Use that to measure the voltage and see if you can provide more of the information I mentioned. Let's see if we can get that TV back to work again.