Asus R541UJ (X541UVK Rev 2.0), bios problems, no boot devices

Dear colleagues,
I'm trying to repair this laptop but I found myself stuck in a point.
The owner came to me saying that the laptop didn't start everytime, sometimes it goes normally, sometimes it goes directly in the bios and some other times it has only blakcscreen.
When I tried to start it, I had only blackscreens, the power light fired up, I saw some black shades changin on the screen but nothing else.
This mainboard doens't have ram chips soldered.
I inspected the board with magnifying lens and made many measures, I didn't find any short circuit but I found that many rails came up, all but VGFX (VCCGT) , VRAM (FBVDDQ) and VGPU (NVVDD), about the last two, I thought that they would be activated in a following time by a driver so I didn't give them too much importance.
For diagnosis purposes I then started to heat (at around 150°C) some spots on the board and heating one corner of the i3 made the system start correctly, I then let it cool down and again it didn't start....heating up the same corner made again other correct starts (I tried 6 times and everytime the same reaction), so I thought that there was some cold joints under the CPU package, and since it is the fourth time in 6 months that an Asus laptop mainboard with 5th to 8th gen Intel Core CPU to have cold joints problems, I prepared the board to a complete reflow.
While rotating the board it happened that seeing it from another angle, I discovered a heavily oxydized spot right in a hidden side of the RAM slot, it was the pin #260 connected to M_B_DIMM0_SA1 signal which had not a solid connection to R1702 so to +3VS.
I then cleaned the oxyd, resoldered the 260th pin and the board started correctly everytime.
I thought it was finished but the another problem came out, because the board can't find any boot device...
It sees correctly the SATA drives (DVD and HDD/SSD) connected to it and even the usb pens with windows or linux OSes, but the board can't start any OS from that devices, so everytime it starts it goes directly in the bios.
In the bios settings, I see other strange things, such as :
Total Memory : N/A (going into Advanced setting with F7, it sees 4096MB correctly)
VBIOS Version: N/A
Graphics : Intel 520 N/A (on this board there's also an Nvidia GPU)
Besides that, I can't change any setting but the time, and everytime the system restarts (selecting "Save and exit"), settings are back to the previous status.
If I try to change Secure Boot setting and installing the Asus factory Keys, it throws the message :
Secure Variable Update is locked down
Try again after System reboot

I then thought it was a bios problem, I tried firstly to update it through the EZ Flash utility, it started to update it, it asked for a reboot but at the restart the bios was the same as before (original it was 302, I tried with 310).
I unsoldered the bios chip (GD25Q64BSIG) and cleaned the ME portion to the same 11.6.0.1126, resoldered it but again the same problem, no boot devices found but all the sata and usb visible in the bios sections.
I then reprogrammed the same chip with the same clean ME rom + the last 310 bios taken directly from the Asus website (copying the necessary serial numbers from the original bios), the new bios was exactly 310 but it had the same boot problem.
Then I thought that the culprit was the GD25Q64BSIG itself, because many other times Gigadevices chips were faulty, then I reprogrammed the same "310 bios + clean ME" file to a fresh new Winbond W25Q64FV, but the same problem again.

Another very strange thing is that everything seems to work good in the graphical department (even VGA and HDMI outputs) but I have zero volts not only on VRAM (FBVDDQ) and VGPU (NVVDD), but also on VCCGT, and I can't explain to myself why there is an image on all video outputs if no GPU is feeded
I have a suspect that the ME is kind of joking me, maybe its internal ROM is really damaged and it doesn't even accept a "bios-side" cleaned ME image...and since I can't boot any device or OS, I can't execute the command "fpt -greset" in order to force the ME to re-initialize itself and accept the clean image.
Now I'm stuck at this point, and I ask you some help or some suggestion, thanks in advance

No advices colleagues?
This is what I see in BIOS section...as you can see VBIOS , Total memory and secondary GPU are N/A

Hi master Vogelek23,
thank you for the interest in my question, and for the clear explaination of the GPUs workings.

Vogelek23 написав:Quite simple to explain - all these video signals (eDP, HDMI, VGA) come from the bare CPU. nVidia GPU is only activated under heavy 3D load, and Intel GPU is activated under moderate 2D load - this is why you have no VCCGT/NVVDD/FBVDDQ power with all video outputs active).

In the past I've never seen other GPU or systems working like this, I've always found at least some little voltage on VCCGT (or IGFX, i.e. Intel GPU) on mainboards with only Intel or with Intel + AMD/Nvidia GPUs, when the system was only in bios settings, that's why many times when I hadn't that intel GPU voltage, I've reworked the bios binary and successfully repaired them.
For example, two days ago I repaired another Asus laptop (a gaming machine with Intel GPU + Nvidia GTX1050, which had two broken input mosfets) , while I was already working on this Asus R541U, and I wanted to compare voltages between this board and that one, I've measured zero voltage on Intel GPU and the usual 0.88v on GPU core of the Nvidia one, while the system was in bios, so no complex 3D nor 2D graphics load.
Anyway, I will take your explaination for the future , thanks !

As soon as I can make some HD photos, I will post them here, thanks

Hi colleagues,
sadly I couldn't make any HD photo of this mainbord beacause of a lack of professional camera, but I started a deep inspection with a big amplifying lens as suggested by the Master Vogelek23, but I didn't find any other oxydation .
I then tried another bios binary written in the same GD25Q64BSIG as it was downloaded, without touching its serial numbers, and the system started up correctly, displaying every info that were missing before in the main bios section (all but the Nvidia part) and started to boot every device containing an OS (sata and usb).
Then I discovered that the lack of the NVidia infos and of course the same miss in the windows peripherals (and the lack of voltages on FBVDDQ and NVVDD) , was due to the partly-wrong version of this bios that I used , because it was for an X541UAK, instead the repaired one is an X541UJ, but every of them belong to the same MLB code, i.e. X541UVK, same physical board but populated with Nvida (the UJ version) or no-Nvidia (the UAK).
Then I realized that there were some errors in the original bios (nothing to complain about the ME part of it), right between the hex addresses that contained the serial infos, that in this bios are repeated around 40 times, which I blindly copied and pasted before from the first address found till the last one, with every other data in between.
I then downloaded another bios dump made for the UJ variant, started a boring copy&paste of the fourty "serial, MAC-address and windows key" from the original dump to the downloaded one, cleaned the ME and written it in the original bios chip, then the system came back to normal operation...to verify that really everything was ok, I updated the bios to the last 3.10 version by the included EZ-Flash feature , and it completed correctly.

So I have to thank Mr Vogelek23 for the support and I hope that this experience can be useful for other members.

Have a nice day!