The thing is that your questions aren't really very applicable. For example if there is a high speed and low speed CAN bus or just one really does not matter cause of details about how it works. Also that applies to the question about read/write/modify - that's not really how CAN bus works at all, it's more like commands and there are some nifty ways to filter, like codes needed. For yesterday's article it boils down to modules not doing careful enough verification and likely a really boneheaded in hindsight mistake in the Uconnect software. The talks will be great resources for a lot of people, I'm eagerly awaiting them.
For example if there is a high speed and low speed CAN bus or just one really does not matter cause of details about how it works.
It matters a lot. Typically, there is a gateway (in body or chassis controller) between the two (or three) buses. It puts some messages from the controller bus onto infotainment bus, but not the other way around. It's done specifically to prevent infotainment systems interfering with workings of the car.
If Jeep has everything running on the same bus, it's incredibly stupid.
That is a bit over simplified. In practice more is passed, but for example the ECU ignores based on rules like RPM above X, TCM not in P, and so on. There is pressure to move to a single high speed bus and modules that pass messages based on priority and time.
