"1a. Why didn't Kṛṣṇa really prevent the Mahābhārata war from taking place?"
The perception is wrong. Krishna & PAndava tried their best to prevent the war to happen.
As discussed in this answer, the premises of Mahabharata was completely one sided, i.e in favour of PAndava-s. The bottom line is that irrespective of Draupadi's insult & other treacheries, Kaurava agreed to return the kingdom after 12 + 1 years of exile. This is mentioned in SabhA parva:
Sakuni then said,--'The old king hath given ye back all your wealth. That is well. ... On the expiry of the thirteenth year, each is to have his kingdom surrendered by the other. O Yudhishthira, with this resolution, play with us
This is a clear breach of treaty & trust.
If an employer doesn't pay 1 month salary, then workers go mad. Then just imagine about absolute humiliation in an assembly, worst insult of wife, followed by 13 years of brutal exile. Everyone has a threshold of "How much is too much".
"1b. Was the war really worth it?"
Despite all above, PAndava-s were still resilient. After the 13 years of exile, mostly they were so humbled, that PAndava-s had hardly any interest in attaining the kingdom as before they did.
Yudhishtira originally made the offer of just 5 villages (that's too for their own livelihood). I have covered PAndava-s view on war in this answer.
In those times, people were not "jack of all trades" like today. PAndava-s were good with rulership as being brought up in such family and that's what they could do to sustain. Some more quotes from Yudhishtira on his distress, if not given 5 villages:
...Kinsmen and friends and Brahmanas shun a poor man as birds avoid, O Krishna, a tree that beareth neither Rower nor fruits. Even this, O sire, is death to me that kinsmen shun me, as if I were a fallen one like the breath of life quitting 'a dead body. Samvara said that no condition of life could be more distressful than that in which one is always racked by the anxiety caused by the thought--I have no meat for today, what will become of me tomorrow? ... It is said that wealth is the highest virtue, and everything depends on wealth....
The number of deaths in the war seems inflated. World war 2 is the greatest war with highest calamities. Even if it's otherwise, then also PAndava-s gave way more leniency to Duryodhana compared to Allied countries ignoring treaties broken by Hitler's Germany.
"1c. why not show the same viśva-rūpa to everyone in the battlefield and stop the war?
The last nail to coffin from Duryodhana was, attempting to capture Krishna who was a peace-ambassador. Jengiz Khan had ruined a country in middle east when his peace ambassadors were killed.
Krishna did show his universal form in the assembly as discussed in Udyoga Parva:
Krishna said: From delusion, O Suyodhana, thou regardest me to be alone, and it is for this, O thou of little understanding, that thou seekest to make me a captive after vanquishing me by violence. Here, however, are all the Pandavas and all the Vrishnis and Andhakas. Here are all the Adityas, the Rudras, and the Vasus, with all the great Rishis. Saying this Kesava, that slayer of hostile heroes burst out into a loud laughter. And as the high-souled Sauri laughed, from his body, that resembled a blazing fire, issued myriads of gods, each of lightning effulgence, and not bigger than the thumb. And on his forehead appeared Brahman, and on his breast Rudra. And on his arms appeared the regents of the world, and from his mouth issued Agni, the Adityas, the Sadhyas, the Vasus, the Aswins, the Marutas, with Indra, and the Viswedevas. And myriads of Yakshas, and the Gandharvas, and Rakshasas also, of the same measure and form, issued thence. And from his two arms issued Sankarshana and Dhananjaya. And Arjuna stood on his right, bow in hand, and Rama stood on his left, armed with the plough. And behind him stood Bhima, and Yudhishthira, and the two sons of Madri, and before him were all the Andhakas and the Vrishnis with Pradyumna and other chiefs bearing mighty weapons upraised. And on his diverse arms were seen the conch, the discus, the mace, the bow called Saranga, the plough, the javelin, the Nandaka, and every other weapon, all shining with effulgence, and upraised for striking. And from his eyes and nose and ears and every part of his body, issued fierce sparks of fire mixed with smoke. And from the pores of his body issued sparks of fire like unto the rays of the sun. And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, and Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya, and the Rishis, possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion.
"2. Gāndhārī asked Kṛṣṇa the same question (in the title); what was his response to her?"
The curse of GAndhAri & Krishna's initial response is discussed in this answer. Besides that,
The holy one said, ‘Arise, arise, O Gandhari, do not set thy heart on grief! Through thy fault, this vast carnage has taken place! Thy son Duryodhana was wicked-souled, envious, and exceedingly arrogant. Applauding his wicked acts, thou regardest them to be good. Exceedingly cruel, he was the embodiment of hostilities, and disobedient to the injunctions of the old. Why dost thou wish to ascribe thy own faults to me? Dead or lost, the person that grieves for what has already occurred, obtaineth more grief. By indulging in grief, one increases it two-fold.
"3. What exactly is Kṛṣṇa's view of hiṁsā or ahiṁsā? When to use hiṁsā and when to use ahiṁsā? Does Kṛṣṇa represent hiṁsā or ahiṁsā? Did Kṛṣṇa classify the Kurukṣetra war as hiṁsā or ahiṁsā?"
As discussed in this answer, "Violence" (Himsa) & "killing" are not 1:1 related. "Violence" is a kind of "violation" of Dharma, while "killing" is merely an act. A criminal being killed by police may not be a violence, however peaceful protesters being shot by police can be termed as violence. With that, the question of MahAbhArata being Himsa or Ahimsa becomes moot.
Krishna & many others consider it as a Dharma Yudh. If we consider Draupadi's understanding of Dharma as correct then, not killing a "killable", is also a sin.
O Janardana, that sin is incurred in slaying one that deserveth not to be slain. So there is equal sin in not slaying one that deserveth to be slain.
Violence is not related only to killing. As we seen above, killing can or cannot be violence. At the same time abusive language is considered as a violence, as discussed in this answer. The same was believed by Mahatma Gandhi as well, that violence can be done with action, speech and thoughts. In fact, due to twisted meaning of "non-violence", he also told following:
I do believe that, where there is only a choice between cowardice and violence, I would advise violence... I would rather have India resort to arms in order to defend her honour than that she should, in a cowardly manner, become or remain a helpless witness to her own dishonor.
Krishna considers "Non-violence" as part of knowledge (gyAna).
BG 13.8 - Humility, un-pretentiousness, non-violence, for-bearance, sincerity, service of the teacher, cleanliness, steadiness, control of body and organs; ... is spoken of as knowledge