One thing I don't understand - if Dhritarashtra was never crowned king, how was Bhishma oath bound to fight on Duryodhan's side? He wouldn't even have been beholden to Dhritarashtra, much less Duryodhan.
Bhishma was oath bound to protect the throne of Hastinapura. When the partition took place, the Pandavs were given the territory of Khandavaprastha, Hastinapura remained with the Kauravas. At the time the MB started, Duryodhan / Dritarashtra were representing Hastinapur. All that the Pandavas wanted was their Indraprastha back. They did not stake their claim to the throne of Hastinapura.Had the battle ended in a stalemate, with neither side gaining an upper hand, perhaps status quo would have been restored, with Hastinapur going to the Kauravas and Pandavas regaining their Indraprastha. But as it so happened, the Kauravas were routed and the Pandavas found themselves with both Hastinapura and Indraprastha.Also, Pandu abdicating didn't mean that his lineage forsook all claims to succession or the throne. In fact, at that time, kings, when they abdicated, usually crowned their sons the ruler and retired. Only reason Pandu couldn't do it was that Yudhisthir wasn't born as yet, and Pandu had no idea about Karna's existence.Even if Pandu had known about Karna, he would have had to been formally adopted into the Kuru lineage by Pandu. In any case, Pandu abdicated the throne before Yudhishthira was born. I have mentioned this in my previous post.If Pandu had had a younger brother fully qualified to ascend the throne, that brother's son would have been the legitimate yuvaraj. Yudhishthira's claim would have been by -passed. But as things stood, Dritarashtra became a nominal ruler- and hence the question arose- who indeed was the true successor to the throne- Yudhishthira or Duryodhana? The son of the ex- king or the son of the present -ruler?