FIY :
I had a long discussion with a senior apple expert about that on the apple repair center online,
I explained my situation in detail.
He is surprised that verbose mode and online mode won't work, because they are the first to be called, even when the classical EFI boot is corrupted.
So he would not be surprised if I had a hardware problem anyway ...
which would be consistent with your kind returns as NVRAM reset is not working for me.
Still, he declared that apple cannot reprogram EFI at all ...
So I have only two option left :
1) logic board replacement with certified apple store repair center
2) EFI chip reprogramming with a dump of an official firmware
I think I will go for the second option as I have nothing to loose