Loki's motives are often murky but he's always been seen to care about his brother. His overall arc is wobbly but it makes sense that they would end up where they do.
The side characters in WW are humanised, yeah, but is that what we want? Except for the Scotsman, none of them should realistically be there, and yeah, we get scenes where we find out more about them, and they're real people with problems and so forth, but that's where it ends. It doesn't bring that much more to the movie. There is not much pathos in Ragnarok, but the characters are all fun. Valkyrie could have been used to say something more poignant about the nature of war or survivor's remorse or something, absolutely, but at the end of the day these people are making me laugh.
As for the action, yeah, I didn't expect anyone on the team to die at any moment. Thor loses an eye though, and Hulk is possibly stuck in that form for ever. Even if you disregard that, do I want to believe in a real threat to the characters? Not necessarily. An MCU movie is never going to subvert the genre much, because these are the movies that set the rules of the genre. Not many supers are going to die. That doesn't make their fight not fun. The arena scenes are hilarious and well choreographed. In fact, most of the action is played for laughs, because these are comedic scenes.