The router is not defective.
It's a question of the own view point ... does your router start with its firmware as expected? If the answer is "no", it's defective ... obviously/at least the software, not necessarily the hardware.
I would like to load an executable to the Ram:
EVA isn't ADAM2, I would suspect, that the format is a different one (but I only know the source code of AVMs "flash partition scanner" from the kernel package) ... and as you could see already, your box is able to run a firmware image, if it's loaded to RAM. What's the idea behind experiments with other formats (as described in the linked page)?
If this image would not "panic", it could continue with other activities. But the original firmware is (explicitely) made to initialize the network interfaces (that's built into the kernel) and even if this hurdle has been taken (anytimes in the future, if your attempts were successful in a manner, I can't imagine in the moment), the "/etc/init.d/E03-flash_update" script tries to install the running system to flash memory.
Let's think a little bit further ... after you've had any success installing the firmware to NAND flash and try to restart the device afterwards. What will you do, when the bootloader code isn't recognizing the NAND flash anyway?
Why do you think, installing the firmware to NAND flash would change anything within/for the "EVA loader"? You've understood, that this code was stored in the SPI flash, haven't you?
If you want to overcome the point, where two of your serial log files showed the same "kernel panic", you have to use an own image ... with an own kernel, too. Do you have such one? Did you understand, how it works and how you could build such an own image?
These are the "basics" I've meant some posts above. You're now mixing up (in my opinion) different ADAM2 implementations (OpenWRT has an own page for "EVA", better read this one) and try obviously some approaches, whose senses I can't realize.
Your problem is clear ... the bootloader was erased by accident and the newly installed EVA version is for a different model and does not recognize the installed NAND flash chip. But it has to read from there the system to start and this not only once ... no, on each further reboot.
So your first task is to get and install a functioning bootloader ... any further steps are (relatively) senseless, if the loader is unable to read the system and none of your current attempts (as documented above) is appropriate (in my opinion), to change anything from this situation.
I did not read any comments or results regarding the proposal, to replace/correct the "device specific block" at offset 0x580 of the loader partition ... instead you're trying some (in my opinion useless) approaches to run a system from RAM. As long as you haven't a plan, how to overcome the "kernel panic", what do you want to achieve now with the other activities?
3) I would like to initialize the sytema step by step.
Which "steps" are meant here? First, second, third ... describe your intentions and your actions, together with the results. But think and research, before you start further actions. If you're not sure, what's the aim and whether the used manner is a right one, you'll put more damages to the device and your chances, to make your mistakes undone, are sinking more and more.
At the first glance, the recovery program for version 151.06.52 seems to contain an update for the EVA loader, at least according to the strings found around offset 0x01554e00 from this (Windows PE) file. Why don't you try now to extract this loader from the recovery program and to flash it (with a correct configuration block for your device - which you have to build first, no question) with your external programmer, as you did it with the 7490 code?
THIS would be the next (logical) step, at least in my universe and I can't see any sense within an attempt, to make the second (or third) step in front of the first one and to try the installation of a system to NAND flash or to run a system (at least not an original one, which panics after some seconds) from RAM.
If you've a full-functioning EVA loader and the installed NAND chip is detected properly, the next step may be taken ... but only then.