My marshal colleagues will tell you that if you work trackside the only safe ('ish) way of working a live track is with a safety car (or in the case of LM three safety cars.
On the whole, at every level, drivers do not respond well to lights, flags or dancing girls, they only slow down when forced to do so by a safety car.
In the UK a couple of years ago we tried a "safety car flag" which was a flag that was shown at which point the leader would take on the role of "safety car" and slow the field down. It failed disamally and was withdrawn very quickly.
As a race interrupter, the quickest way to get an incident cleared, track rebuilt etc. is by throwing a red flag as the trackworkers can work quickly without constantly watching their backs - given that this isn't a great way to run an endurance race the safety cars are the next best option.
Even then at LM we saw a number of incidents where cars went off under the safety car (the 44 spinning at pit entry comes to mind!) so pro drivers or not (and of course a vast number of them aren't) I can't see that you could retrieve vehicles, repair barriers etc. without physically ensuring that the cars slow down and if the only way of achieving this is with safety cars then that's how it should be.
Given the conditions at LM I doubt that the safety cars were having difficulty keeping the field at a suitable pace, it didn't appear to be like that from the screens.
Working stragegy under the safety car is something for the teams to manage and often differentiates the good teams from the excellent teams and with three cars on track there were three opportunities to get on the end of a train. To me the safety car is something that goes with racing, you can't recover cars, drivers etc. from a live track so somehow you have to manage it so that this can be done safely - or you throw a red.
As far as I know there are yellow lights around the circuit that back up the flags, I can check but I don't have first hand knowledge of this.
Whether the use of the safety cars at LM was appropriate or not is something I don't have any information on.
Edited to add - often on a long circuit there are "jobs" waiting to be done around the track that are done whilst the track is neutralised, it's not just the area that the main incident is in.
