The LE installer expects to have exclusive use of whatever disk /dev/device you install LE to so it does not support dual boot installs (and we have never offered to support dual-boot setups, it's too much hassle). However if you understand how to fiddle with partitions and create space for another OS and configure things (maybe, or maybe-not?) then it's completely possible. You can mod the syslinux config (or install grub if you or Ubuntu prefers it) and configure that to create a boot menu offering LE and Ubuntu. For a remote to work with that it probably needs to be the kind that acts like a wireless keyboard since there is no 'OS' at boot time to do fancy remote support. If the remote emulates USB keyboard arrow key navigation and USB is active (should be at BIOS level) it should work. There are probably threads in these forums on all those topics if you go (re)search for them. Again, we don't support dual-boot so there's an element of self-initiative required.
https://wiki.libreelec.tv/configuration/…es#lirc-support <= We dropped the lirc on/off from the settings add-on some time ago (wiki needs to be update) but AFAIK everything else documented there should still apply and lirc can be used if you really need it. Without knowing what other special things you are doing in Ubuntu it's hard to say whether it can be easily replicated in LE without old-school lirc, but in 99% of cases there's usually and easier way. Give detailed descriptions and share code/scripts involved (often more useful and a non-technical users bad description) if you want better guidance.
NB: HDR support requires an HDR capable Intel or AMD GPU (not nVidia) and the LE11b1 Generic image or newer. This uses GBM/V4L2 graphics which means no VNC. If you want to do things remotely you can SSH in use "kodi-remote" to do basic keyboard navigation around the GUI from an SSH client. If you really want VNC then you need to use the Generic-Legacy image, but that uses X11 and thus means no HDR. You cannot have both.