Thanks a lot for the tests!
The MCE USB dongle could receive something but unfortunately it doesn't look like valid IR signals so we can't use it to get the details of the Xbox remote protocol. It was worth a try, though.
I managed to deduce parameters from the ir-keytable -t output though and had a shot at using them in the xbox remote driver. Can you please test with this image (for RPi2/3)?
libreelec-rpi2.arm-9.0-devel-20190323102559-d446ddb.tar
When you test with this image please keep an eye on "double-action" button presses (i.e. if a short button click results in 2 or more movements). I've added a bit of headroom to the parameters but if they are too strict they could (occasionally) happen and we'd need to increase the headroom.
so long,
Hias