Seska didn't want to leave Voyager. She thought she was helping them. Once she was forced to leave, her goal was to take Voyager for herself. She wanted the ship. If she was smart, she would've come to the conclusion that they were never going to get home, so any rules or treaties are pointless.The Federation and Cardassia were allies.
There was no reason for Seska to have acted like she did.
Legally Janeway had to welcome Seska with open arms, and protect her from any of the Maquis who might have felt betrayed.
Her Cardassian rank, if it was high, might have forced out lieutenant junior grade B'ELanna Torres from her position of Chief Engineer, or for Tuvok to give up one of his jobs. Tactical and security are two jobs.
Along with Best of Both Worlds, Scorpion and the Gift, Dark Frontier, and Q-Who, which were the 2nd through 8th episodes on the aforementioned list.Because there is no real build-up to the finale it feels like. So, you can watch it without any real concern of missing too terribly much. The finale is a pretty big spectacle in its own right, so you get the larger action set piece with some time travel shenanigans and the Borg. It;s kind of like Star Trek comfort food.
It seems like the more obvious explanation is that people like the Borg action episodes. Unless you think people go through the thought process of "I'm going to watch Endgame today, or Best of Both Worlds, because it's comfort food, and I don't have to know what's going on."
Endgame is no more a "standalone" story than All Good Things...