I think that problem is we are trying to see things from a 21st century perceptive. In MB times, Dharma meant more than anything. Mistakes of Pandavas are present in MB. But they are never presented as mistakes. They are either due to circumstances or destiny or because some elder told it or so. Mahabharata always made clear everything Pandavas did by some or the other dharma. Look at Dyuta sabha for example. Yudhi says that he played acc to dharma because it was his dharma to not refuse when summoned to play. He again says that it is sinful that Shakuni is playing not Dury, but still he would play as it us destiny or something that I don't remember now. Does that make any sense in today's community? But it did make sense, long ago. Such is each action of Pandavas. No one questioned killing of Nishadas because no one cared about them. Let me not begin with Pandavas cheating in war. It is always justified by DS events.😆 Like, Bhima while taking oath in sabha often forgot Draupadi's humiliation which happened a few minutes before, but remembered very well while kicking Dury's head. In Adi Parva, we can see how Bheema as a child enjoyed hurting Kauravas. He used to push them down trees, hold them underwater until they were about to die, break their heads with stones etc. These are never mentioned as sinful. But when Dury, who could not take that anymore, tried to poison Bhima, he immediately became a sinful wretch. Bhima did more harm than Dury did. But MB is partial to Bhima there. This is always the case. All mistakes of Pandavas are well covered by Dharma. So they go unnoticed by a staunch, traiditional person. And given the levels of Dharmic bhashans in this book, anyone in ancient times would have admitted Pandavas were the purest rather than going through all the pain.
Most of the times, these stories have been repeated through the mouths of rishis etc. So if you sit before a sage who is telling this, you might ask why the difference in treatment? But a person from gone centuries would never taken the risk of posing himself/herself as a jerk in public. That was the custom. Also these stories were passed orally for some 3000 years. It was very unlikely for any interpolator to remove every single thing from a poem as big as this one.