I have that lock file in my LR folder and having not issues. I think you may be correct updating to LR Classic CC from 5.7.1. Not sure if going from Mavericks to High Sierra is somehow on involved as well. That OS is 5 years old.
When I updated from LR6.12 I think it found the catalogue on it's own and merged. There were only 2 choices. Create a new one or point to the existing one. It appears even pointing it still didn't recognize it.
Like I said there where posts from people having trouble trouble updating to standalone LR.14 if they missed the version 13 update. A clean start might be the best option.