If I play Hour of Wrath on Haldir, could he use his Combat Action to attack before enemies attack, and still not exhaust? I understand that if enemies attacked, he could attack each enemy once with Hour of Wrath, but because his Combat Action says he exhausts to work, I didn't know if Hour of Wrath could negate the exhaustion part since it is called an attack. Given the wording, I lean toward him exhausting, but not sure.

You will still have to exhaust him. Hour of Wrath removes the exhaustion requirement from a standard attack, but it doesn't allow you to avoid paying the cost for a card effect-- which is what Haldir's ability is.

Even if you could do this without exhausting Haldir, however, you still could only use his combat action once. The text says 'limit once per round.'

Yep, understood his Combat Action only works once per round. So this works as I suspected. Thanks!