Re. the taxi - true. As far as I know, nothing was found in it to indicate they had triggered anything.
“they could have found much better positions to park ( like up on that slope where they could see )” I’m not sure what you mean here. If they had anything to do with the triggering (even acting as spotters), I would agree their location would not be ideal. But setting of an IED is a crime of opportunity. Nobody knew when the Marines would be coming down this road since they tried to vary their routes. Just because someone isn’t in the prime position doesn’t mean they wouldn’t take that chance. The taxi occupants may have been insurgents readying to take part in later combat. They may have been additional spotters for the position of the convoy. They may merely have been in the wrong place at the wrong time and had nothing to do with anything.
It’s interesting that we’ve heard nothing from NCIS on their background. Where did their families live? What was their occupation? Did NCIS ever check to see if they were enrolled in the college that they were allegedly returning from that morning? Also, as far as I know, their families weren’t paid solatia payments (compensation for unintended casualties). There’s a black hole of information on the taxi occupants from both sides (at least available to the public).
From what I’ve read it looks like the Convoy passed the “Safe house” on the way to Haman’s group, spent some time there, then went back. This probably happened pretty regular, and there are only so many roads you can take.
A good bet would be someone from the safe house saw them go down, ( on a motorcycle perhaps ? ) and went up to position himself if the convoy took that route going back. Might have repeated it a dozen times waiting for the chance. They would have known exactly the best place to be. The taxi spot was just about the worst place to be.