Also, does DKIM really provide the same sort of authentication that PGP / s/MIME would? All DKIMs verify is that the message was in fact authorized to emanate from that domain - PGP & S/MIME attempt to confirm that a particular identity belongs to a particular address.

DKIM signs the message headers (including the From: header) and (a hash of) the message body. So you can validate that a message has not been tampered with, and is from the mailbox it says it is (assuming your mail provider rejects messages with an invalid signature, irrespective of e.g. DMARC).