The UBPorts image resides in the same main partition as Debian, so you don't need to delete either. The bootloader has extra boot options, and by using my script (attached) on your Cosmo alternative OS installer SD card thing you can flash the UBPorts boot.img to one of these boot options and essentially triple-boot Android, Debian and UBPorts.
You'll also need to put the boot.img on your SD card in the same folder and change its name to 'ubports-boot.img' for my script to work, and also add this:
echo "UBPorts,$MBF/Cosmo_Installer_UBPorts.sh" >> $INSTALLER_FILE
To the bottom of the list_installers.sh file (above the 'rm -f $PARTITION_FILE' line) and, as noted above, put the ubuntu.img file in the root directory of your Gemian install. Now boot to recovery with the SD card inserted and install UBPorts to another boot partition of your choice (But not the second one, as noted above there's no networking there!)
I've found the UBPorts image to be the most fleshed-out alternate OS yet - the camera works, keyboard layout works, 4G works, Bluetooth works