Ichikawa Jr. only wanted to climb up the corporate ladder, and he is under his father's thumb, that's why he's in an official relationship with Tadokoro.
Ichikawa Senior will obviously find out about any affairs, because there's a HUGE merger at stake. Both Ichikawa Sr. AND Tadokoro let Jr have his affair, as long as the merger can happen.
Tadokoro is using her status to keep Jr, and just like so many Japanese women out there, she's taking the attitude of "as long as he comes home". This is actually, literally taught to daughters in Japan.
Unless Ichikawa is secure and confident with his own capability to survive outside his family's name, he won't strongly push the break up. Narita, being the other woman, will always be the victim. She got transferred because the affair was found out. If she stays there, she will never get to any higher position.