I did some marshalling (Think Bike marshals) in the late 2000's. At a few races I was on the leading bike for the Elite Men. I was very fortunate that the route mess never happened, but at a race or 2 (fortunately I was marshalling later groups then) the routes were changed by the organisers. This was done WHILE the bunch was racing. The only people informed were in the lead car. The lead bike was informed and then usually fell back close to the "change in route" point and the lead car took over. BUT, typically a TB marshal would then stay behind and wait for a "foot" marshal to take over the point. One of the risks discussed often by the marshals was what would happen if the lead car was not reached?The elite riders trust the leading vehicles (whether car or motorbike). So Emma following the lead car was the accepted operating procedure.
I do however agree that Emma should have turned around and gone back when she reached the danger tape. The lead car made a major mistake here by going through the tape and the driver should take the blame for the mess at that point.