Thanks a lot, again, for testing!
From the system/udev side everything looks fine, these were the 2 lines I was looking for:
eventlircd_enable=true
eventlircd_evmap=default.evmap
So the problem most certainly resides in libudev or eventlircd. Unfortunatley eventlircd is very quiet and nothing was logged to the journal that could provide additional hints.
Before I go down the rabbit hole of digging into eventlircd/libudev - which could take me a while - there's one last thing I'd like you to do: Could you do a quick test with the latest milhouse build, using your existing rc_maps.cfg config, and see if eventlircd picks up the input device there (post the output of "lsof | grep /dev/input")? That build contains a newer systemd version, so if it's really a libudev/systemd issue there's a slight chance that could already be fixed.
Builds are available from here: LibreELEC Testbuilds for RaspberryPi (Kodi 18.0) (just pick the latest, currently #0802). But better use a separate SD card for testing - going back to 8.0.2 will result in a mess because several databases, addons etc will have been updated to newer versions.
As for KEY_ENTER/BACK being slower: that's normal and caused by the long-press support in Kodi, configured via the default keyboard.xml file:
<return>Select</return>
<return mod="longpress">ContextMenu</return>
<enter>Select</enter>
<enter mod="longpress">ContextMenu</enter>
...
That feature allows you to map different functions to a key/button, depending on if you press it shortly or longer. That feature is only available on keyboards, not on (remote-) events that come in via the lirc socket.
For a full list of keycodes supported in Kodi best look into the source code (the keyMap definition at around line 120): xbmc/LinuxInputDevices.cpp at Krypton · xbmc/xbmc · GitHub
so long,
Hias