1. It is Unmaking, so it is matter 5.
2. Yes, it is lasting, and so is Self-repairing machine (SRM); self-repairing machine repairs 1 structure per success every 15 minutes. This spell blocks repairs up to a successes repairs.
So, if Mage A casts SRM on a car with 5 successes, the car repairs 5 structure every 15 minutes (for ever). If Mage B casts Stop-the-Machine and gets 2 successes, then that car would then be repairing just 3 structure every 15 minutes. If Mage B casts Stop the Machine and gets 5 or more successes, the car loses the self-repairing property.
3. The ability to block the repairs simply makes what remains of the object much harder to repair magically. Casting reassembly on a broken vase of structure 2 and getting 2 successes = the vase is as good as new even if most of it is missing.
Casting Reassembly on a broken vase that has been treated with Stop the machine needs to beat the Stop-the-machine property.
Casting Repair with potency 4 on a truck that has received 4 structure damage, repairs 4 structure damage.
Casting Repair with potency 4 on a truck that has received 4 structure damage that has been treated with Stop the machine (2 S), repairs just 2 structure damage.
All in all, Stop the Machine gives a property to the item that makes it much harder to repair magically, whether through Self-Repairing-Machine or with other spells or powers.
That doesn't stop the object from being damaged more or being damaged again; it just makes it much harder to be repaired with magic.