Something to remember, too.
BoaF can be used anywhere. Boulders? You can't throw them indoors. Likewise, you can't throw them at someone who's near a shield grog or sodales.
Nope. Attack Advantage for Aimed Spells do not carry over into extra damage. This was brought up here and discussed. IIRC, David Chart himself confirmed it is just a "to hit" roll. I remember that very much precisely because of Novus Mane.
Here: Building Finesse in to spells/devices - #39 by David_Chart
Of course, but we're talking of thrown rocks.
If bigger rocks are thrown, on a flat trajectory, with less force than smaller rocks, we can, at least for game balance purposes, assume that the 2 cancel out each other: You gain +5 because of size, you lose 5 because of lack of strength.
If going ballistic, so that the rock's fall will increase damage, he'll lose in precision, thus cancelling his aiming bonuses.