Controlling display backlight manualy

Yes, it is completely safe. Echoing into /sys (or /proc) does not overwrite kernel configuration directly, it runs a kernel userspace function which do validate the input.

xrandr/xbacklight actually performs pretty much that (as far as I know).
Since it worked before I bet that xbacklight is simply finding the wrong backlight directory in /sys/class/backlight. I suppose there is more than one directory in there, right?

I'm not 100% sure of it but

xrandr/xbacklight will choose the acpi directories (e.g. /sys/class/backlight/acpi_video0) before trying something like /sys/class/backlight/intel_backlight/ (which is the one your current driver is creating/using).

(I'm not 100% sure but found a decent reference on arch wiki that more-or-less confirms it)

To force xbacklight to use the right directory you can add the following to xorg.conf (or preferably drop a *.conf file in /etc/X11/xorg.conf.d/) with the following section:
```
Section "Device"
  Identifier  "Card0"
  Driver      "intel"
  Option      "Backlight" " intel_backlight"
EndSection
```
(Disclaimer: I have not tested it since I do not have an intel card, if this section does not work you might try substituting "Card0" for "Screen0")

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