As far as E, not sure you could do more damage than a box truck would do. If you tried to fly into a crowd somehow you would crash and probably only hurt people within a relatively small radius.
With flying cars, you'd never be able to guarantee the safety of any outdoors event (and in either case, it doesn't have to be the result of terrorism, a drunk driver or a midair collision can still cause these problems)
Anti-air artillery is not a reasonable civilian defense system. It's just silly, not in any way relevant to the discussion at hand, and you know it. Stop trolling and move on.
How about remote shut off and control by a gov body? Too expensive and a wonderful vector for remote hacks.
FAA tracking and interception on deviated courses? Holy crap I doubt they can scale fast enough if buying and using flying cars "takes off". See what I did there? Also interception will lead to the same fire ball of death problem.
No fly airspace, which would become nearly every metro and suburb the same day. Thus making the flying car useless.
Unless we do SAMs or antiair, the flying car will be dead in the water. And still would be. It's too large of a premeditated and accidental disaster precursor.
If they're k12 rated. Most bollards are not k12, especially in cities due to shallow dig depths. Even if the bollard or planter themselves are k12. Normally they're never dug deep with supports because contractors are lazy assholes if you don't stand over their shoulder. And those that are k12 are expensive as shit. I actually work in protective design. That, and I think I'm funny so :P
One thing to keep in mind about the threat of "flying car attacks" is that weight requirements of staying in the air generally remove the "mass" part of the equation.
Someone could pack a flying vehicle full of explosives, but explosives that pack a big punch and that are compact & light weight are pretty regulated. Explosives that are easy to acquire tend to have more weight (fuel oil & fertilizer, for example). You could only pack so much in a vehicle before it couldn't take off anymore.
In general, I don't think flying cars would be any more of a risk than light aircraft, which have been around for decades without very many incidents of successful terrorism.
As an example, about a decade ago, and anti-tax nut flew a light aircraft into an office building that housed local IRS offices in Austin. It caused a lot of damage, but not serious structural damage and there were no fatalities other than the pilot.
Let's just add, oh, I don't know, 100 pounds of a fertilizer bomb like the Oklahoma city bomber. He used a lot more, but 100 is still crazy dangerous. Add you have a vehicle that laughes at bollards, gates and fences.
You do know, bad people actually do exist. Trusting tech companies and scifi hopes and dreams really isn't an effective strategy.