LE mounts the second partition on the SD card as /storage and uses this for persistent data, e.g. Kodi saves to /storage/.kodi - the rest of the filesystem is decompressed from the SYSTEM file on boot and is read-only so it's hard to mess things up.
See https://wiki.libreelec.tv/configuration/…ration-advanced for instructions on creating and configuring a custom IR keymap. The remote will use the rc6 protocol so there might be existing keymaps that work, but it's normally quicker to transcribe one from scratch than test all the existing ones trying to find the one that exactly matches (there are lots of them).
It's impossible to guess what might cause Kodi to get stuck on the logo without seeing log files. If there is an issue in the OS it might be visible from the UART output. If there's something in Kodi .. you can enable persistent logging via the LE settings add-on and then check the logs (or look for crash logs) on the next clean boot.
CE and Armbian should be able to boot using the same u-boot, but their boot files are differently named, e.g. CE uses dtb.img and kernel.img so you might need to rename their files to match LE conventions. Updates would also not work, although GXL devices are no longer supported yb CE so there will be no updates. Armbian will be different again. I'm not sure what changes bumerc has hard-coded in u-boot, but as long as you can boot to the u-boot console via the UART cable changing the scripts stored in the u-boot environment will always be possible.