Hey! Long time reader, first time writer now
First of all: i hope JB will get well soon!
As for the question slowing down under yellow/SC wouldn't it be possible to mandate every driver to go only half the speed they did in the previous lap under double yellows? It is possible to track the speed every car does at a certain point on the track. This data can be sent automatically to race control (as it is already sent to the teams).
Then for example if a driver took a turn with 100 kph the previous lap (lowest speed counts) he may take it at a maximum of 50kph during double yellows. Could be automatically tracked by race control and with the proper it equipment teams could easily tell their drivers the allowed max speed. The main hardware (tracking position and speed of cars) is already there, all you need is the proper software that calculates allowed max speeds for the drivers.
Then implement harsh penalties: 0-5% speeding drive trough, 10-20% 5 or 10 grid penalty, more than 20% disqualification.
With the proper equipment speeing infractions can be seen by race control in real time and penalized right after.