"How did Shuro think he could marry Falin when he hated her brother?" you have to understand. Toshiro is from a whole different country. In his head he and Falin would move back to the Eastern Archipelago and they'd see Laios twice a year tops. You can pretend to get along with in-laws you don't like for a few days a year, people do that all the time.

The actual flaw in his plan– which shows he doesn't really understand either of the Touden siblings– is the fact that if the plot hadn't happened and Falin had for some reason said yes to his proposal, Laios would have packed his bags and moved away with them instantly.