Again, any time two cars contact each other at highway speeds, it is a potentially deadly situation. Many people have no idea how easy it is to lose control of a vehicle at highway speeds after a contact. It’s a surprise for at least one driver, who may react by violently swerving away from the contact, instigating a bigger crash. If the cars are not exactly parallel, they may start to rotate. So if you can safely move over, you should.
Even slowing down can be dangerous, because if the car is paralleling you, slowing down may cause him to hit the front quarter of your car instead of broadside, and if that happens there are good odds that at least one vehicle is going out of control. If you are in his blind spot and he starts to change over, braking is the right thing to do. If you are almost parallel, you would have to panic brake to get out of his way in time, and maybe not then. Panic braking at highway speeds is also a good way to instigate a crash.
As for processing whether the shoulder is safe in a split second - you shouldn’t. Part of defensive driving is to have that information at hand already. You should be paying attention to the road, including the condition of the shoulder. Of course, we all fail at this sometimes, but it should be the goal. Defensive driving is all about situational awareness.