You can move upto twice speed for a trample
A trampling creature can move up to twice its land speed as part of the trample. FAQ
You can move about as you trample. the only restrictions are that you can enter the square and any creatures in those squares must be 1 size smaller than you or smaller.
Power attack wouldnt apply. Its a special attack without an attack roll
You could smite evil as a swift action and then trample it. The bonus to attack wouldnt affect but the bonus damage would i think affect the target of your smite but not other evil foes who happened to be also trampled