SAKAZAKI. I think it's pretty obvious. He makes money selling important information and the rebels knew everything in advance. Sakazaki might be selling them info and double crossing Mikhail (he surely got a big chunk of cash from Mik by selling Sudou). He betrays without remorse whenever he has the chance.
If the boss was someone else unrelated he would have no problem disposing of Mikhail given the opportunity. Even though Yuri is a psycho, it still won't be that easy for him to kill his nephew.
On the other hand it's worse for Fei, because Yuri has the knowledge of Mikhail's feelings for Fei. If the boss was someone else unrelated, he wouldn't have known that. So now the rebels can use this info to get to Mikhail.
Dear, fan service remains fan service, it concerns the main characters or side. This is exactly what bathing, sex scenes, food and almost everything that happens on the island are. For the development of the plot and the relationship is little concerned and is created mainly to ensure that the fans rejoiced and drooled.
Yeah, dude, every Saturday they chase together to the disco (-‸ლ)
http://pic1.mangapicgallery.com/r/album/a5/raw_/270411_3690318.jpg