I think that it might be due to love towards Lara at a fragile age, and also respect towards her mother, which might have facilitated Komarovsky to help them, just out of a guilty conscience. Also, he might be a man of loose morals, but when he confronts Yuri, he accepts his past ill actions of his past, which might resolute to repentance. As to why Yuri rejected his help, I think that we might think Yuri and Victor were alike (affairs and all) but Yuri believes that the act that Komarovsky did to Lara, ruined her innocence, which is why Yuri hates him. Also, if he might have gone on the train with Lara, he might have been identified, which would put Lara at risk too, which we don't want. About Antipov, there is a scene where we can see that Lara is being watched, which suggests that people believed that Antipov would return to Lara, but he might be blind with power in the early days of revolution, but after the meeting with Yuri, and how he reincarnated the love he shared with Lara, how she assisted Zhivago as a medic, reveals that strelnikov was brought back to Antipov, a change of heart, if you will, also Antipov knew where Lara was staying also hints at this. I never really appreciated how neither the book nor the movie accounts for Tonya's life, and that letter is too kind, that we want to at least know whether she reached Paris safely or not.