I find it humorous that they mention artillery firing directly, but they make all of it effectively indirect with the minimum range requirements.
Some artillery (infantry mortars and rockets) only fires on a high parabolic path. Other, tubed artillery can fire a high indirect path or a direct fire attack without a minimum range. The petard mortar being a turret mounted gun with a minimum range is rather silly for me as an ex-artilleryman.
WW2 short barreled assault guns were capable of both direct and indirect fire, with indirect fire having a possibility of a minimum range, but direct fire being something they could use to defend themselves, or bring fire directly on a nearby target.
The petard works within the game, even if it makes no sense as other than an oversimplification.
Not firing artillery from within a building when they don't acknowledge direct fire as other than self-observed indirect fire makes sense.
Not firing down through a roof from outside artillery works as a simplification that doesn't require rules to figure out how solid a roof is.
Giving cover to a unit on the edge of a building from artillery would work, except that it contradicts the artillery ignores cover ruling. Of course, while I don't have the original rulebook, the artillery ignores cover ruling only exists in the FAQ from what I have.
Cover helps against blasts, but not always. An air burst can ignore a lot of things that would protect against a ground burst. Corner cover would be no help against any artillery or grenade burst. Artillery ignoring cover is the quickest way to deal with the variables, so hopefully they'll amend their answer to not giving cover to a unit in an entrance attacked by artillery.