Probably everything has access to everything on the CAN bus.

IIRC car manufactures are irresponsibly lazy and never properly air-gap infotainment from critical stuff.

Even if this were true (and as another commenter says, it's not) I don't see how that fills in this gap. It would make sense if the first step were "MCU goes crazy," but not with a simple reboot.

There's a firewall between the CAN bus so it cannot be affected at all by the MCU.