How high can the debris loft after a particularly nasty collision?
Two satellites hitting at an acute angle should produce a cone of debris in front of them, of which about a third deorbits and a third goes up into a higher orbit that's an average of the two.
Two satellites that hit at an obtuse angle, well, they pancake and produce a donut of debris. The stuff headed straight up is on a parabolic orbit that will hit the atmosphere on the way back down, but in the meantime any other satellite that crosses paths with it is effectively hitting a wall of stationary debris, creating a new cone that ladders up higher. Is there enough space that the ladder stops, or does it just keep building?
After a collision, the periapsis is at most as high as the point of collision, and the apoapsis is at least as high as the point of collision. In plain English, if two satellites collide with each other at an altitude of 450km, it doesn't matter what the original orbits were; 100% of the debris will have a low point in their orbit at or below 450km, and 100% of the debris will have a high point in their orbit at or above 450km.
One of the quirks of debris deorbiting is that drag is exponentially higher the lower you are in orbit, and any drag that the debris experiences at any point along its orbit will manifest in a reduction of altitude 180 degrees along is orbit on the other side. So if you have a piece of debris with an eccentric orbit, let's say 300km at periapsis and 1,000km at apoapsis, after a fairly brief amount of time you'd expect the debris to have a periapsis of 299.9km and an apoapsis of 500km. Then perhaps 299km periapsis and a 350km apoapsis. etc. I'm making these numbers up but you get the idea: the high point in an orbit is the part that drops the quickest.
Even if you were to launch 100,000,000 full sized satellites into orbit at 400km altitude and deliberately orchestrated Kessler syndrome, space would be unusable for a few years, but would be 100% back to normal after 5 years.
The dangerous orbits are those in the 500-2,000km range. Satellites whose orbits never bring them low enough to experience significant atmospheric drag. Those are the satellites this new rule is targeting.
I don't think debris 'going higher' isn't much of a problem. Whenever this happens the orbit is going to be more eccentric - meaning a lower periapsis, and consequentially lots of drag that will cause a rapid deorbit.
On the second point about parabolic orbits I also find that probably relatively low risk because we are only talking about a fraction of an orbit for a collision to occur so unless the debris field was massive the chance of another collision is probably still low. Remember when we are modelling orbit collisions normally we are often talking over 25+ years - 100,000 + orbits.
I think the main problem is busy orbits (e.g. sun-synchronous polar orbits at popular altitudes) where most of the debris remains roughly in the same orbit following an acute collision but has a lot of other potential collision targets. Also as satellites are disabled by a collision they lose the ability to avoid other objects already in the same crowded orbit - i.e. the fraction of objects able to take avoidance decreases increasing the chance that future collisions are from 2 incapacitated satellites, removing the possibility of avoidance.
Two satellites hitting at an acute angle should produce a cone of debris in front of them, of which about a third deorbits and a third goes up into a higher orbit that's an average of the two.
Two satellites that hit at an obtuse angle, well, they pancake and produce a donut of debris. The stuff headed straight up is on a parabolic orbit that will hit the atmosphere on the way back down, but in the meantime any other satellite that crosses paths with it is effectively hitting a wall of stationary debris, creating a new cone that ladders up higher. Is there enough space that the ladder stops, or does it just keep building?