Is crossing a black hole's event horizon possible?

[PeterDonis]

If I were to observe any effect of crossing the horizon on Earth, I would retrocausally gain information about the future behavior of the shell

The word "retrocausal" might be confusing some people. A better way to put this would be that the fact that you can't know in advance the entire future behavior of the shell (and the spacetime in general) is why you can't observe any local effect when you cross the event horizon. Or, to put it another way, the event horizon is not locally defined, it's globally defined; to know where it is, you have to know the entire spacetime, including the entire future.