You’ll only see DevEUI and AppEUI for an OTAA Join Request. For “regular” uplinks, you’ll not see those, but will see the DevAddr instead. The DevAddr is not unique; the server needs to know the secret NwkSKey for each device with that DevAddr, to pick one by comparing the MIC.
The Network ID can be derived from the DevAddr.
I don’t think that there’s any retransmission flag; a node just sends a new OTAA Join Request if it did not receive any Join Accept, or retransmits the same uplink if it did not receive an acknowledgement for confirmed uplinks. See the specification (where TTN uses LoRaWAN 1.0 at the moment).
Finally: beware that you might be receiving plain LoRa packets, which are not encoded using LoRaWAN. You cannot tell until you validate the MIC, for which you need to know the secret NwkSKey. The online decoder will happily show you all kind of details which are basically bogus if it’s not even LoRaWAN to start with. See its example that specifies the secrets, and then play with the payload (like boldly change the first byte into 00) or play with those secrets.