Could you check and confirm the version of Lightroom you are using?
Also, please copy the files from the camera to the computer and then try to import it.
Let me know if this helps.
Actually it is very Straight Forward to revert to Yosemite if you did a time machine backup or used a drive cloning software just before you upgraded to the newer version of OS X.
Apple has never and will never put in place a system to go back to the previous version of OS X like Microsoft does for Windows. But if you use Time Machine or one of the drive cloning piece of software it is very easy.
Try taking this up with Apple as it is their problem not Adobe's.