It's been a while since the last post but I thought to give an update on the E4418B repair.
The unit is fixed (almost) and here is the story
I went back to my original troubleshooting notes and noticed that what was stuck on my mind, and I wrote in the posts here, was actually not correct. The CSBOOT pin of the CPU which is the CE of FLASH chips is indeed active but the CE of U11,U12 SRAMs is stuck at LOW (disable) and CE of U39,U40 is stuck at HIGH (enabled). I checked again and confirmed my notes were correct. That indicated to me that the CPU is trying to read from/write into the boot SRAMs U39,U40
The SRAMs were cheap, so I replaced U39,U40 and, lo and behold
the unit turned on and passed all self tests
I think what threw me off the right path was the behavior of the E4418A repaired by Shahriar on his youtube video. In his case the same two SRAMs were bad but the display was turning on and there was a message from CPU so I was expecting that my unit is stuck at an earlier stage. However, this is E4418B with a different firmware and I guess the behavior is different.
Now the problem is that the keypad is also bad, the REF LED does not turn on even though the voltage is correct and the power on button does not work either. I could not repair the keypad so I ordered one from china. Hopefully that will be the last issue with the unit.
TheSteve had told me in the beginning to change the SRAM and I did't listen