at the meeting with Lucas, why did Robert Taro not tell him?

I agree 100%. For me, it was this exact situation that transformed what could have been an engaging drama into an irritating soap opera. There was absolutely no reason for Robert to keep that secret after he learned the truth about Lucas. He was not married to his current wife when Lucas was conceived. He did not know that Lucas even existed, and could not have abandoned him. In so many ways, Lucas was already a beloved member of Robert's family. The "secret" could have easily been divulged (and celebrated) without anyone being hurt, and Lucas could have lightened his emotional baggage by knowing that Robert had not been a loser as a father. What was even more insane was the fact that Lucas had worked closely in politics with Robert for so long and knew what kind of man he was. Why did he love his mother so much when she kept him in a closet as a child and made him eat food from a bowl on the floor like a dog? As soon as she was released from prison, she went right back out and held up another bank. I was saddened by the fact that the writers depended upon this "twist" to attempt to make the story interesting. With such an outstanding cast, beautiful locations and huge budget, they could have gone in any different direction. By the end of the movie, I detested everyone. These days, I only change trains from the TGV to the regional line in Marseille. I used to love going there and the series did a great job of reminding me how lovely she still is. I adored walking down those streets and watching all the newly arrived people moving from place to place in the bright sunlight, still wearing their traditional costumes. It is a city like no other. I struggled to watch the whole thing. At least I added lots of new slang to my vocabulary.

Since you started with questions, I have another one that really puzzles me, I hope it's no spoiler. I've seen it yesterday all the way, and, when you've also done, help me to see if I got it right.
The old witch (feel free to change the initial at your will) met Robert when she was severely injured, with a bullet in her head and a dissected carotidis. Robert, then a medical student, was called from Paris to rescue, but he couldn't help so he had her transported to the hospital, where she was first saved then jailed. He never saw her again.
DID HE *beep* HER WHEN SHE WAS IN A COMA?
Maybe she was right being pi$$ed for so long.