who is dollar's father in last segment

The same actor who portrayed Zhang throughout the entire film portrayed Zhang in the last segment. The actor was made to look older and heavier, but the actor was the same actor.

Dollar never said he was parentless. He said he had no mother. He did not remember his mother's name and was too embarrassed and ashamed to admit in front of the class that he did not remember his mother's name, so he said he had no mother, then joked that he was a test-tube baby.

The film did not present a resolution to Liang's situation. We don't know if Liang survived chemotherapy. We don't know what happened to his wife and son. Liang met with the mine manger-turned-welder because he wanted to borrow money from the mine manager. He needed money for the chemotherapy. When the manager informed Liang that times were bad, the mine was losing money, small mines were banned, and he became a welder (which meant he was on the verge of losing his manager job, which also meant undergoing a massive pay cut) and was planning to work for China Petroleum laying pipelines in Kazakhstan and had to invest 30,000 yen in the move, and had to borrow the money from his family to make the transition to Kazakhistan, Liang realized the manager had his own financial problems and did not ask to borrow money from him.

We don't know if the father really became westernised. We do know he changed his name, but we clearly see that he never learned English and most likely hanged around other people from China with whom he made business.

Knowing how little communication there is between the father and the son, you can always say Dollar is parent-less- the father cannot communicate with him, they are immersed in different cultures, they barely talk, they do not understand each other and do not even make an attempt (except for the translated conversation at the end) to speak.

Aside from that, it is the same actor playing Zhang in both segments.

In the beginning there was nothing, and it exploded.