This answer is as per the Mahabharata Tatparya Nirnaya of SriMadhvacharya. He read all the Mahabharata recensions found across India, which he writes about in the Tatparya Nirnaya, and clearly explains what constitutes an interpolation and which parts are authentic, along with the correct interpretation.
First of all, the Pandavas never went to hell. Because what Yudhishthira saw there was an illusory hell. So, let me briefly tell you the story. When the Pandavas were going to Svarga, the youngest one would first perform dehatyaga, and then the eldest. So, in the end, Yudhishthira was remaining. He, along with a dog, was going. Suddenly, a chariot came down for him and asked him to come in the chariot. The charioteer asked Yudhishthira not to bring the dog with him. But Yudhishthira said, “I won’t come to heaven without this dog.” And then, the dog turned into Yama Dharmaraja. That dog was actually Yama Dharmaraja. He, along with Yudhishthira, went to heaven. On going to heaven, Yudhishthira saw that Duryodhana was sitting on the throne.
Yudhishthira was astonished and asked the devatas, “Why is this evil Duryodhana in heaven? He hasn’t performed anything good in his life. So, why did he who didn’t listen to Krishna’s words reach heaven? Where are my brothers? Bheemasena, Arjuna, Nakula and Sahadeva, Karna, Draupadi?” At this, the devatas sent a messenger with Yudhishthira, who took him to Naraka. That place was very ugly and unclean. It smelled very bad there. He went there with the messenger and heard familiar sounds: ‘I am Bheemasena!’ ‘I am Arjuna!’ and like this. Yudhishthira was shocked upon hearing this. He said, “If my brothers and my wife are in hell, how can I go to heaven? I won’t go to heaven. I accept hell if my brothers and wife are here.”
Then Yama and Indra came there and said, “What you saw now was an illusory hell. It is not real. It was shown to you because you had committed Adharma by not believing the words of Krishna Himself.” Actually, in the Mahabharata war, Krishna asked Yudhishthira to say, ‘Ashvatthama hatah’, because Dronacharya would put down his weapons only when his son died. So, Yudhishthira doubted Krishna’s words and thought it would be Adharma. So, that’s why he is shown the illusory hell. Yama and Indra further said, “Your brothers are in heaven. Duryodhana, due to his punya prarabdha, is present in heaven. After his punya prarabdha ends, we will push him to the eternal hell, where he will experience endless sorrow and will never come out of it.”
So, when Yudhishthira went to heaven, he saw his brothers sitting in their divine forms. Draupadi was sitting along with Bheemasena. Out of love for his wife, Yudhishthira went to touch Draupadi, but Indra came and stopped him. He said, “Draupadi is actually Bharati Devi, the consort of SriMukhyaprana Vayudeva, who is your brother, Bheemasena. Draupadi was your wife because she had Shyamala, the wife of Dharmaraja, within her. And that Dharmaraja is you. So, Shyamala within her was interacting with you, and the rest slept. Similarly, she had Shachi’s presence within her. So, Arjuna, being Indra, interacted with her, and the rest slept. Nakula and Sahadeva, being Ashvini devatas, interacted with their wife Ushadevi, who was present in Draupadi. So, Draupadi is mainly Bharati Devi.”
Note: This is the presence of 5 souls in Draupadi, namely Uma, Shyamala, Shachi, Ushadevi, and mainly Bharati Devi, who is even mentioned in the Garuda Purana.
Indra and Yama further said, “You four brothers have the presence of Mukhyaprana Vayudeva, because He is present as breath within you. Plus, He is even present in different forms within you. So, the level of presence of Vayudeva was there in you; the attraction of Draupadi towards you was also that much. Bheemasena was Vayudeva Himself. After Him, Arjuna had the greatest presence of Mukhyaprana Vayudeva in Him. Now, that presence of Vayudeva’s form in you is not there. Hence, you cannot touch Her now.” Yudhishthira realized it and then went to Yama. Yama hugged Yudhishthira, due to which the remaining human traces in Yudhishthira went away, and he became equal to Yama Dharmaraja in qualities and merged with him (his moolaroopa).
Similarly, Bheemasena, Arjuna, Nakula, and Sahadeva went in their moolaroopas, like Mukhyaprana Vayudeva, Indra, and Ashvini Devatas, respectively. Draupadi entered Her moolaroopa, which is Bhaarati Devi. So, the Pandavas weren’t sent to hell literally, but it was just an illusory hell shown to Yudhishthira for his Adharma of not believing in Krishna’s words. So, you can notice how subtle Dharma is.
But why did Krishna tell Yudhishthira to say Ashvatthama Hatah?
It was because Dronacharya, Bhishmacharya, Krupacharya, etc., are Krishna’s devotees. And the Pandavas too were devotees of Krishna. Dronacharya was a Brahmin and used to follow Silouncha Vritti (which means he wouldn’t ask anyone anything, even if he is in trouble, or whatever happens to him). Then also, he participated in the yuddha.
Plus, Dronacharya had sinned by being on the opposite side of Krishna’s devotees, thus opposing them. Secondly, he was also sinning by attacking the devotees of Bhagavan. So, Krishna was like, ‘You are my devotee. You must not be allowed to commit so many sins.’ Thus saying so, Krishna asked Dharmaraja to say, ‘Ashvatthama Hatah,’ but when Yudhishthira added, ‘Kunjaraha’, meaning elephant, after ‘Ashvatthama hatah, ’ Krishna blew the conch shell so loudly that the Kunjaraha was not audible to Dronacharya, and he thought his son died. And thus Dronacharya left his weapons, and Drishtadyumna beheaded Dronacharya.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!