The 1976 film was a good film, told the story well in spite of the insipid subplot. It will be had to find a better Admiral Yamamoto the Toshiro Mifune!
I have no memory of the subplot. I assume it was simply a pretense to get us to the battle scenes. That was an impressive cast, rivaling even A Bridge Too Far.
Mifune IMO was robotic and it was hard to take Arnold from Happy Days, Pat Morita, as Adm Kusaka.
The idea of his Son being in love with a Japanese girl does not sound like a bad idea for a sub plot. It was just totally screwed up in the movie.
In Tora, Tora, Tora, instead of a sub plot they would just shift from American story, to Japanese.