1) I've updated the xow package & it should now come with the modprobe.d conf file as template(!) where you have to edit the commented lines & the service file should hopefully now work properly:
xow: added modprobe conf & fixed service · SupervisedThinking/LibreELEC-RR@a632084 · GitHub
It's probably not the final stage, ideally I start the xow service after some service detecting some sort of usb devices.
Once you've enabled the xow service by systemd enable xow it should run automatically even after reboots. I won't enable them by default because this is completely an opt-in & someone who builds the image himself can easily enable the service & delete two #
2) thx for looking for the package, I already suspected it's gtk3-system and it's probably something like Bad error when gdk-pixbuf-pixdata is not in path (#525) · Issues · GNOME / GLib · GitLab & I have to look into it why glib-compile-resources doesn't pick up the toolchain gdk-pixbuf-pixdata which should be in place.
Upstream LE has the same problem & patched it out gtk3: disable use of to-pixdata during build · LibreELEC/LibreELEC.tv@86f4306 · GitHub