Regarding the different tank attacks:
with preserve random seed, this should not matter - but think of rivers and so on, this changes combat results.
However fighting another battle and THEN doing the "important" one can change the outcome dramatically. This is some sort of cheating. But there is no difference if you attack with a veteran tank or a redlined regular tank first, the seed and the random numbers and your attack/defence values are always the same. (this is a comment to binyo's "For instance u have 2 tanks attacking an infantry.
The result by switching which tank is going to attack first will give different results.")