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 > these events are sent to identified pubkeys?

On the most basic version, yes. Clients can use aliases as well if they want additional privacy. And those aliases can also be independently registered with relays or simply used to write Spam reports or deletion events. 

> What if the recipient's client can't decrypt the event

It doesn't matter if they can or cannot decrypt. If they are listed as a pTag, they are the only interested party in the event. If they are reporting spam, it is spam and can be deleted.