I was wondering if there is a "rule" or something that should prevent from themed elements covering part of a ride system (like the pillar in this photo) due to safety and maintenance?
Coaster track vibrates and moves more than you'd think. You could probably cover a support like that but I wouldn't connect it to the rock work. You don't want a giant vibrating rock every time a train passes through.
The coaster and mountain are two separate pieces in Everest.