[the_3mp3ror]

 Is Coreelec something that can be installed on Android from an APK, like SPMC can be?  

No, it's a very lightweight OS that basically just runs kodi. You can flash it from an SD card on most Android devices, but I think you need a rooted device. It's very easy, you find instructions on their website. Libreelec is basically the same. For what kind of device would you need it?  

NVIDIA Shield, but somewhere in the house I have an old  MX8S+ Android box that I guess that I could dedicate to Coreelec. 

That's what I did, installed coreelec in place of Android on one of these TV set boxes, I never use it for anything that is not Kodi, anyway.