#1 The barn can't be destroyed
#2 Yes, it does any good to attack truck convoys - for each truck you kill, the downtime of resupplyable items (town, guns, ords, radar, barracks, fuel tanks)
which are already down at that moment increases by 0.9 minutes.
Example: The town had been bombed at a base, with 30 minutes of downtime remaining on the town buildings. If you wipe out one truck convoy (8 trucks), you will increase that downtime by another 7.2 minutes.
Convoys appear every 10 minutes.
#3 Same mechanism as for trucks, only that trains do resupply factories, not bases.
#4 (Not asked but I will answer anyway
) Barges are working all the same.
And your wish is?