Gosh I suggest reading at least SOME of the press articles or other topics about the halo! The halo is not intended to have anything to do with impacting recovery vehicles, head protection was being tested years before Bianchi's accident - a forward roll hoop was suggested in 2012 for use in 2013!!!
The three criteria they tested were car-to-car, car-to-object, and debris-to-car. Debris includes small and large items, but particularly nose boxes and wheels as those have caused high profile fatalities. Car to object is head on collisions where cars are buried in the barriers, e.g. Kovalainen (who received a concussion in Spain '08) and Sainz in Russia, or like Viso's crach in GP2 when he rolled over a wall in 2007. Car-to-car includes incidents like Wurz-Coulthard, Schumacher-Liuzzi, Alonso-Grosjean, and Raikkonen-Alonso. They used historical data and tweaked the conditions of the crashes to simulate hundreds, maybe thousands, of different scenarios! The also chucked millions of small items at the car, showing the driver was 17% less likely to be hit by something like a spring - even though that wasn't a design consideration.
The halo is capable of withstanding enormous loads, so much so the crash tests mean a significant improvement to the chassis strength - so the halo deforms before the chassis breaks. I forget the load it has to withstand, but it's something like 16x the static weight of the car - crash test loads like this are unprecedented in F1!!
"There is one big friend. It is downforce. And once you have this it’s a big mate and it’s helping a lot." Robert Kubica