Hello again

So, here is the text from the Vibro Cannon:
Vibro-Cannon: This is a smaller version of the Titan Tremor Cannon, placed on a small anti-grav platform for
mobility. When a vibro-cannon fires, draw an imaginary line (2 cm wide) from the cannon to the target (this may be
direct or spotted). Any buildings along this line are destroyed if they fail a basic save at -3. The actual target is hit on
a 5+ and any troop stands or vehicles along the line are hit on 6+. If one vibro-cannon hits the target the TSM is 0, if
two hit the TSM is –1 and if three lines cross the TSM is –2. Separate detachments that fire at the same target do not
add in this way. The attack ignores shields and any target that uses a template is hit on a leg or track location.
And here is the Unicorn:
Unicorn: Striking quickly deep into the enemy’s ranks is the essence of the Eldar way of war. To this end the
Unicorn was built as a direct response to the perceived slowness of the Vibro-Cannon. The Unicorn is a
modification of the normal Falcon hull, mounting a Vibro-Cannon on its main axis.
When a vibro-cannon fires, draw an imaginary line (2 cm wide) from the cannon to the target (this may be
direct or spotted by a command unit). Any buildings along this line are destroyed if they fail a basic save. The actual
target is hit on a 5+ and any troop stands or vehicles along the line are hit on 6+. If one vibro-cannon hits the target
the TSM is 0, if two hit the TSM is –1 and if three lines cross the TSM is –2. Separate detachments that fire at the
same target do not add in this way. The attack ignores shields and any target that uses a template is hit on a leg or
track location.
Several questions:
1) Why does the Vibro Cannon on the ground mount make the building make a save at a -3, while the Vibro Cannon on the Unicorn mount make the building have a basic save?
2) Since there are only 2 Unicorns in a detachment, and separate detachments do not add in to the TSM, how can 2 unicorns cause 3 lines to cross?
3) Why does the Vibro Cannon need to have a unit spotted by a "unit", while the Unicorn says a "command unit"?
4) Do we infer that if the Vibro Cannon (on either mount) is firing at a spotted unit, then it deviates as other artillery?
Thank you,
Jake