The greatest risk would be bribery of his security personnel to provide a "backdoor" for an assassination. If that were the case, it definitely wouldn't look like one of the scenarios you described. It would be obvious that it is a targeted assassination.
FWIW: I'm Brazilian, have lived in both Rio de Janeiro and São Paulo and have had known many people with private security and armored cars there.
In today's world you need the media to treat him as a martyr to make him one in the general public's eyes. There is currently a smear campaign against him stating things like, "he is no longer just a journalist, but actively aiding and abetting the enemy", "aiding a fugitive", "helping to commit treason", gay with a green-card-denied-gay-lover, etc
The knives are out. He would be a martyr to a few that cared. The ambivalence in the masses to this subject is truly scary.
Greenwood is being treated like Applebaum and the other Wikileaks personnel surrounding Assange. They are viable targets as they are aiding dissenters that are determined, effective and dangerous to the status quo.
I don't actually believe that the US is going to try and remove Greenwald from the picture now, but not for the reasons you state.
The dam is breached so he is no longer a real threat. The usual process now is to control the response, public opinion and any senators that might raise any opposition.
They are very very good at it; well practiced and effective. You don't go up against the combined resources of the United States government and expect to win. This is like a gnat attacking an elephant.
If Greenwald strikes any more than a flesh wound on the US government then I'd be surprised. They are already back to business as usual and now with the added cart blanche, which is the knowledge that the public know they are being spied on, but the NSA now know that most of them don't give a shit, as long as they can still read status updates on Facebook.
GreEnwald is exposed down there in Brazil. A petty street robbery gone bad, a car accident, or a simple "sudden" heart attack, could all make this into a nothing event.
People in this kind of position have habits of "committing suicide" from the "stress":
http://en.wikipedia.org/wiki/David_Kelly_(weapons_expert)
http://en.wikipedia.org/wiki/Death_of_Gareth_Williams
I would put money on the act that Gareth Williams worked on, or knew about both Tempora and Prism.
http://www.theregister.co.uk/2012/05/03/gareth_williams_inqu...Maybe he was Snowden part 1: the failed mission.
End conspiracy theory...