I did miss this section in my skim, but it's also the first place I went as well. It doesn't seem like you could ever be sure that no old messages would be sent with an earlier timestamp if you don't know the set of nodes that are participating in the protocol.

Moreover, if you allow the set of nodes you do know to make operations maliciously (e.g. a single bad node enters the set and tries to cause conflicts), things get much more complicated, and I don't really think you can escape the impossibility results in relation to consensus protocols...