It would make sense to email the company producing the race and ask what their policy is, for whatever scenario(s) you have in mind. There are some really good questions that have already popped up here that I'll put my RD hat on to answer related to the races I produce...
--What if vandals remove or change course markings and there is no lead vehicle/bike? In that case the course map is the course I'd expect racers to follow.
--What if vandals remove or change course markings and there is a lead vehicle/bike? In that case all racers follow the lead vehicle/bike. Lead vehicles/bikes need to know the course as well as the RD, and need to be in the loop regarding any "on the fly" changes. In my races if a lead vehicle/bike gets to a place that is mismarked they are instructed to phone the RD immediately and the usual instruction I give them is "fix the marking and continue on the correct course per the map". Sometimes we'll even pull someone out of that vehicle to act as a Course Monitor until a more visible correction to signage can be made.
--What if a fellow racer ahead takes a wrong turn? Do not follow that racer unless instructed by race officials. If there has been a legit course change it is on the RD to get somebody there to instruct racers what to do.
--What if I am directed off the official course by a Course Monitor or lead vehicle/bike? My advice runs counter to the "runners must be responsible to know and follow the course" rule that we all hear from day 1 running XC in middle school. Very rarely there will be a legit reason to send a runner onto an alternative course, namely an emergency on the course or something dangerous has happened on the official course that we are detouring rares around. If a CM or lead vehicle goes off course follow that instruction. At worst, if that CM/vehicle was wrong, you have a legit claim against an RD for any prizes you lost by following his staff's instructions.