Or it could be the mark from Jenny's broom...
Hehe. Jenny basically just swatted her with the bristle part, so she wouldn't be bruised, but she could definitely have some broom dust.
In terms of your post, if you actually want to have Aorle ask Rollick why he didn't have sharpshooters in place, this can happen off-camera, and the explanation will be this:
The building interior does not have cover to hide them. This is deliberate, because that cover would be easier for Panterras and potential cronies to use against our side (bearing in mind they use magic) than for us to use against him, and Rollick doesn't like that kind of nasty surprise in battle. Rollick wants to be able to see what's coming, rather than having extra evil mages popping out from behind concealing objects.
That means the sharpshooters would have to hide outside, and pick off Panterras as he approached or left the building (and chances are he'll be leaving by magic one way or the other.) Since he does have "protection" against arrows in the sense he'll just teleport when he realizes someone is shooting at him, the first arrow would basically have to kill him instantly, or he'd just leave. If he caught sight of one of them, again, he'd just leave.
Now, it's very possible the nomads are good enough shots to pull that off. But since Rollick has never actually seen them fight anything (and hasn't seen their NPC profiles

) he really has no way of knowing what they can do. Other than their own description of their abilities and maybe seeing them train a little, he really has no idea whether they'd be a help or a hindrance, whether they'd stick to the plan or decide to do their own thing, whether they panic under pressure (what little evidence he's seen so far would indicate at least some of them do,) or heck, if they could be convinced to change sides. He just doesn't
know them, and something that requires this much precision-- and upon which Kira's and possibly Uluki's life depends-- Rollick is going to stick to people he knows he can trust, not only in a moral sense but in terms of their combat abilities.
If this ambush went wrong, they'd never be able to get Panterras into another one, and then Kira's pretty much dead. The nomads
are definitely potentially useful, but they are also a
very unknown quantity and there are about a thousand different ways they could, intentionally or unintentionally, complicate the plan and mess everything up. And of course, the fewer people who know the details, the less chance someone will let them slip in the wrong place.
They've also had to assume (correctly, as we discovered when Delphina was spotted and followed) that Panterras and friends have at least some eye on the compound, and probably the ambush site. Therefore everyone there
must be accounted for in some way: the assassin, the bard, the traitor, the two victims. They took a gamble on Julen not being recognized, and it paid off, but Rollick would assume it would be pushing their luck a bit to think that half a dozen armed nomad sharpshooters would go unnoticed as they moved into position, and there's no good way to explain them to Panterras. ("Oh, don't mind them, they're friendly!"

) This is stealth, not outright warfare. Panterras would not knowingly join a battle unless he found himself embroiled in it (as happened.)
The potential benefits to having sharpshooters are, in Rollick's mind, far exceeded by the risks and problems they would pose. Aorle may or may not agree with that line of reasoning, but that's what the reasoning is.