Boy do I feel like an idiot, I used OSMC for a while and before that I ran OpenELEC which didn't have Bluetooth support so it didn't even occur to me to look there.
Anyways pairing via de GUI in LibreELEC settings works perfectly, safe and secure too, great :). I got my phone, tablet and Bluetooth speaker paired in seconds (while on OSMC I couldn't get anything paired anymore).
Then to get music playing over the USB sound card all I really had to do was run pactl load-module module-udev-detect. My sound card is detected and is set as the default sink instantly (I guess because it is the only sink available on my RPi2) and my Bluetooth streamed music starts playing over my speakers, yay! At the same time I can play anything using Kodi and Kodi's sound is passed through over HDMI while simultaneously streaming Bluetooth music, exactly what I wanted to achieve.
To automatically detect my sound card at boot I added pactl load-module module-udev-detect to autostart.sh. Funny how that was really all I had to do to get my ideal setup working.
Lastly I would like to note that I tested all this with an old USB sound card I had lying around (a "C-Media USB headset solution") but assuming that any USB sound card will be detected as an ALSA sink (including the PCM2704 USB DAC's) this should work over S/PDIF with those kind of USB DAC's too I guess.
Last question (I promise): all this got me wondering about one last thing regarding Amlogic S905 devices with an S/PDIF output. Is the S/PDIF device a separate ALSA sink that I can configure as the default sink with pactl while at the same time giving Kodi full access to the HDMI output so I can achieve what I described above with an Amlogic S905 with built in Bluetooth and S/PDIF out without any additional peripherals, that would be really cool. Or would I still need an external USB sound card on those devices to use as an ALSA sink for my Bluetooth A2DP music streams?