Yuddhishtira plays the game of dice because he was trying to follow the words of Sage Veda Vyasa, as said in this book. Yudhistira was trying to not fight with his brothers and had no other choice than to play the game and loose everything. He was bound to do it. Your point on the Pandavas not exactly being the sons of Pandu, then we can also argue on the point that Dhritarastra is not the son of Vichitraveerya and has no relation to his "father". The points of Duryodhana being good or bad can go both ways, but ultimately, the cons are the superior ones, making Duryodhana bad. This is like Ravana because even though he was a great Shiva Bhakthi and was a great musician, he did so much evil like taking Sita away from Rama, and other worse things.
Sources: First Source, Second Source.
I hope this clears your doubt.