1: No. There's no bonus to hit for higher level Computers.
2: No. Computers have no effect on damage or accuracy as a bonus/penalty.
3: 70% looks like a raw base chance. This is likely affected by officer skills, speed, range, and enemy anti-cloak measures.
4: I don't think so (if you mean the standard SFC2-like scan that all ships have). Active scan adds about 20 to the range at which you detect units, which is determined by ship size and Computer level.
5: I doubt it, though it probably should since a ship that is "pinging the waters" is more easy to find than one that is running silently.