Spoilers contain spoilery clues on how to get around that bit of mission:
Rocks it is, though - there is indeed another route around the inside of the cave, and they've even gone some way to signpost it for you by plonking a healthkit or armour or something flashing on the floor to the 'left' of the two top-ledge guards as you look at them. Then it's just a matter of following this route, and I'm not sure I even needed to throw a rock as I sneaked past the four guys hanging around in the lower bit of the cave.
Getting the mission reward makes traversing the world feel weird, though. You really have to start your own trouble, even if you've left all the outposts in play, as I still have. I'm going to try and leave them that way until I'm a wedge more through the story, although I have scuppered all the Radio Towers now.
I don't think the San Andreas method of outposts being attacked by enemies would be all that unwelcome, either. I hear what @krushing says about it being a bit of a cheap tactic, but the mechanics of inter-faction battles are already in place in FC3, so you wouldn't necessarily even have to have the attackers winning in every case, just a bit of a contest for the landmass, like a single player, open world game of CoD Domination. It might even help to give a sense of being involved in a larger conflict. It's something Games still haven't managed to do any more effectively than it was done in the original Carrier Command, really.