This is a good question, Indeed, it was made look that Mario and Dimitri didn't brought their guns with them and only have weapon knife. But, to go along with dimitri's plan b, they would have to have a gun. Therefore there's nothing that says that they didn't go back to the car and get them.
I really don't think that it's the general or one of the air force base crew because, it would be unlikely that they'd try to sneak in and he wouldn't be alone that way. yes, they might have the fire power with them, after being summoned to drop his weapon, he would have said something and not just fire at will at the house.
I really don't think it's a biker, cause it's been said that it wasn't one of them, and they would have known if it was.
It's been said that Mario was more used to prepare hits, than Dimitri... But on the other hand Dimitri was ex KGB or something, so he's not totally clueless in those situation. Still, my choice,,,, either someone that we don't know, or Dimitri.
That was far from an admission, he just said that with shadowgod cliffies, you should have a higher limit then that for calling cliffhangers.