And the trick is that you can't just be DOSed by a peer, you can be DDOSed by the peers of your peers of your peers, as I see it.
The most that can be done to infer authenticity of indirect messages is to see whether such a message rejects the authorship of a known previous message having the same handle -- via the SelfChain. In virtually any case of handle collision, this will occur.
Re: floods -- a station only processes messages from a peer. So in fact in all cases the proximate cause of a flood is identifiable, and you can "UNPEER" and "GAG" him.
Flooding by a peer is annoying, but is not what people normally think of as "DDOS" (normally the term implies a flood of rubbish received directly from unauthenticated third parties.)
How liberally to peer -- is a matter for an individual station operator. Peering with every passing acquaintance has obvious down-sides.