Does this waste the attacker's action if the card was nullified by a defender's bodyguard or the same card with the same or higher power?
Yes. It's treated as normal action performed by attacker, however attacker cannot apply its effects and he just does nothing.
Also, when cancelling the effect of the trend expert already applied to the dice by playing a trend expert on your turn (which is the only way to nullify it because you can't do so when the trend expert is originally being played right?), does it have to be the same or higher power?
You cannot reply immediately with your trend expert if your opponent has applied it. You have to wait for your turn. In order to remove trend expert already applied, you can use any trend expert, even with less power. Trend expert action is not hostile action and you don't defend against it. This is why the rule "the same or higher power" doesn't apply here.