What would be their reactions and opinions about the Human-caribou war. Also what would they do to support us?

Celestia: Grateful that her people are safe thanks to the humans intervention. However there are things going through her mind regarding the humans that she'll be concerned about.

Sympathy towards the humans: As everyone knows in war, casualties are to be expected. She'll develop a sympathetic feeling towards the humans that they are risking their lives. In return and in the end, she wants to make sure that all of her people will remember them as heroes.

Hearing humans committing war crimes: Even though she despises the caribou at what they plan to do (Rape, slavery, and all that fucked up bullshit). But when she hears news of humans committing war crimes (Torture, executing prisoners of war, and any non-sexual violent crimes), she'll feel concerned towards the humans that they are destroying their morality by committing one violent atrocity after another all in the name of justice.
Examples:

"Since when are savages worth an innocent life?"

"Fight with me my brothers and sisters! Spill the blood of these savages! Their is no shame. Their souls are already lost!"

What would the other ponies think about it? Will be all black and white or justified?

Il do my idea of how Twilight would handle it:
I think she would be very shocked at first, a all of the "villains" she's faced are fairly tame and not really that destructive. Facing death by another's hand would be very shocking to her along with the caribou intended actions. At the beginning of the war should would see both sides as bad and be very morally confused since her entire world has been clearly Black and White, Humanity's gray will mess her up pretty badly while she comes to terms with justifying killing someone.
Her scientific side would be fighting with her moral side as she would probably take a sick curiosity to the weapons and equipment of both sides.
After the war (Humans winning of course) her entire mindset would be shattered, A war is not a nice thing and is scaring on humans let-alone sheltered ponies. She would definitely face an identity crisis as she comes to terms with herself over what's right and wrong and change from having to deal with death.

So Twilight would be very confused and curious throughout the whole thing pretty much.