I mean, the goal here is to grab the stone while they're disoriented from an artillery strike and book it, not to pull of some sort of unseen heist. yrs has said in the past the the options he gives for situations like this are those the LQ thinks are reasonable based on her level in the relevant skill(s). From the front page, the relevant skill for stealing things is stealth, and the skill for booking it is dodge. LQ does have a specialty in "Sleight of Hand" (i.e. stealing), which means (if I'm not horribly wrong about the mechanics) that she can apply her A ranked advanced stealth skill to this situation. She's not bad at stealing things, and if she can grab the rock then she can rely on her A rank dodge skill to get out of there.
Now, all of this planning is happening under the aegis of her War C skill (yrs needs to update that, see skill gains at end of last chapter), so there is a chance she's misjudging the situation here, although it's likely lower than War C would imply because she's good at stealing stuff and running. If it does go wrong then she gets bogged down trying to defend a hard point in the middle of a group of enemies, with her support 1 - 2 minutes away. Which would be bad, but I feel this is less likely than you're implying.
It depends on how comfortable you are with the risk I guess. Again, it would be a very effective tactical move if it works, plus we get the rock, so I'm all for it.
Skills for reference.
Stealth C (14/25) [C5] • +5 (robe) • Specialty: Infiltration, Sleight of Hand
Vanishing: A (5/40) [A5] :: Stealth • +5 (stealth)
Sable Grace: A (25/40) :: Dodge
War 4d6+8
2, 2, 2, 5+8= 19
30/20 Rank C achieved