Hello again. Sorry for getting back so late to this, I had a really busy time.
I just tried a new update from a fresh El Capitan install, made according to the instructions in OP. Then I lauched the update to Sierra from the Installer in Applications. It took one minute, then rebooted, and almost immediatly started on "Boot macOS Install on <my mac drive>". I just had the time to glimpse one line below "In 0 seconds". So I couldn't activate verbose mode. And it hanged.
So I turned it off with the power button and back on again, and had the three drives in clover, "Boot macOS Install on <my mac drive>", "Boot Mac OS from <my mac drive>", and "Boot Recovery from Recovery HD". Then I had the time to put on the verbose mode, since there was no timer anymore.
Here is where it hanged.
View attachment 214986
After a while, the apple and the time line appeared, with green streeks, as if there was something wrong with the display config.
And after doing a hard shutdown and booting again, the option to boot to Install remained, even after booting to the second option (directly to the previous system).
I had previously tried making the Sierra update by a USB flash drive, booting from the clover on the USB drive, not the one on the ssd of the computer, otherwise it hanged as well. Funny thing was, the usual tools (liks disk utilities and so on) were not available. And the install, which I made on the existing ssd with El Capitan, lasted 29 minutes. At the reboot, it was kernel panic, rebooting automatically, sometimes with the interdiction sign with text saying it will restart soon, sometimes the plain sign. This is the first verbose output I had.
View attachment 214995
Sorry, the picture is not quite complete, I hope it can make some sense however. I made some tries with the restore hd, but it didn't work either, so finally I decided to reinstall El Capitan entirely, and to update from the Installer in the Application folder of the existing El Capitan system.
Here is where I am now.