Wouldn't the Athosians have been left with at least one comm set while they were on the mainland of the same planet as Atlantis?

I'm not sure (obviously, or they would have had one here!). We only really see the expedition with in-ear radios, which I'd assume wouldn't carry round the other side of the planet.

A more paranoid commander than Carter might look askance at Lorne's choice of people to lose track of...

Yeah, but, you know, we never see her go off-world with him. Gotta be a reason for that, right :)