In terms of theoretical possibility don't you add the probability for an or situation.

So for your dice scenario

You want the odds of rolling 4,5,6 on dice one OR rolling 4, 5, 6 on dice 2

0.5 each time so 0.5 + 0.5 = 1

Yes, example one correct.

The odds of rolling 4, 5, 6 on the first roll of a dice OR 4, 5, 6 on the second roll of the dice is exactly the same.

Though a reroll, generally defined as a second chance in the event of failure is resolved differently, The effect this has on the outcome is as follows. First, you have the odds to succeed on the first roll. If you pass the first roll, the second roll doesn't matter. If you fail, though, you get to try again. So, the odds can be described as, the chance that you succeed, plus the chance that, if you fail, you succeed on the second attempt. Mathematically, this looks like (chance to succeed) + (1-chance to succeed)*(chance to succeed). If you want to use algebra, you can also write this as 2*(chance to succeed) - (chance to succeed)squared.