The first Safety Car in Formula One racing was used in 1973 at the Canadian Grand Prix
The most safety cars in a race was 6, also the canadian GP (2011. Also the longest and slowest race on record!)
The fluid isn't interacting with anything until it reaches the combustion chamber, but it if enters the combustion chamber above the speed of sound, via the injectors, well that might cause some surprises like pre-ignition.
Possibly. I certainly don't claim any knowledge here, but empirically, hi-speed flow out of the injectors seems like a good thing to me - Better atomization (?) Hopefully Florian will be along shortly...... Cheers, Ian