"to a creature with which you are familiar"
This could be sticking point. My PCs tend to not be familiar with most corpses. Familiar implies more than cross the battlefield acquaintances.

"if the target is on a different plane than you, there is a 5 percent chance that the message doesn’t arrive."
Hmm. Send duplicates depending on the desired error rate?

Yes, you can use Sending to send a message to a creature, that still exists, and is still a creature, and you are familiar with.