I would be surprised if your approach isn't the same or similar to that of most measurers, G.B. In the two-way traffic on a curvy road scenario, another dilemma occurs when the RD agrees to split the road with cones, we measure on both sides, then the RD doesn't follow through. And I have witnessed lead runners take the tangent in and out of a cone line. In either of these instances, a short course is run. Any record performance would not survive a validation.
Maybe there is nothing that can be done about this for most events. For the 30,000-participant Army Ten Miler in D.C., Bob Thurston and I are hired every year to monitor and supervise the course layout. This is a big and sometimes colossal job. Though all the soldiers who nominally put out the thousands of traffic cones work with detailed cone maps, Bob and I expect to put in two hours of cone work before the start, and sometimes more during the race. In almost every edition of this event, had we not been there to direct this operation and then maintain the layout during the race, the course would not have been run as measured. A couple of years ago, Bob ensured the cones defining one particular 90-degree left turn were placed appropriately. He then moved on to another part of the course. As he swung back by this intersection later, he discovered to his dismay that someone had taken it upon themself to move dozens of cones away from this turn and place them on a different street. A few of the lead wheelchair/hand-crank athletes took the wrong turn before Bob could fix this. We learned later that a "helpful" police officer had an out-of-date course map with this turn going up a different street. Had Bob not been there to rectify this, the race would have been run on a substantially different course. Any records would have gone out the window. Had this been a money race, and had some runners had taken one turn, and some another, this would have been a disaster.
I do not know how many large events employ someone to ensure the course is laid out properly. I conclude that there must be some magnitude at which every large race must have involvement by certifier(s) to guarantee the course run is the same course that is certified.