Responses
I think more or less the same too. :)
No,he just did that to get his way to the top and his mother help him through that
Actually I think Shou had two reasons to do what he did: 1) preventing the pork Chiba from revealing the truth (because that can endanger his own career). 2) having the possibility to obtain roles in the future. His mother helped him (she knows something awful about Chiba's past).
Something bother me a little. Maybe I get it wrong but I really believed that Shou slept with that Shiba only to prevent that pork from revealing the truth (that he is Iwaki's son). Not to get some roles. I didn't think this was his main purpose but maybe I'm wrong. I can't figure that Shou is so calculating. I think he's just a toy in Shiba's hand and most probably in his mother's hand (who seems to be very bitter).