Good, concerning (1) the LEDs, du -a /sys|grep led gave me some interesting output (around 117 lines). After some testing, echo heartbeat > trigger worked well, and echo none > trigger – executed in folder /sys/class/leds/orangepi:red:power/– effectively switched off the red light.
In order to keep it off after reboot, I had to dig a bit and found some helpful hints on https://forum.armbian.com/topic/787-setting-onboard-leds-with-rclocal-or-systemd/
Finally, I managed to find a working solution by creating a file /etc/tmpfiles.d/leds.conf :
w /sys/class/leds/orangepi:red:power/trigger - - - - none
w /sys/class/leds/orangepi:red:power/brightness - - - - 0
w /sys/class/leds/orangepi:green:status/trigger - - - - default-on
w /sys/class/leds/orangepi:green:status/brightness - - - - 1
Setting the brightness of the red LED to zero was necessary to actually switch it off after reboot. Additionally, I tried to reduce the brightness of the green LED (reduced the value from 255 to 1). But I'm not sure, whether it actually changed much. Maybe some other configuration file or tool, I didn't come across, would be more adequate.
About (2) connecting a monitor I'm not sure, whether kernel and hardware work well together. When I tried a Ubuntu 16.04 image before, there was no output either. (In total, I tried two different monitors and cable connections.)
In respect to (3) the keyboard layout, the options dialog in armbian-config simply wouln't open. I wonder, whether it's due to the SSH connection. If I knew more about keyboard configuration, I might adjust it by hand (just to have it ready, in case at a later time monitor and keyboard could and should be used).