I think the Spirits were not wrong.. this is a human conflict and they are the ones getting hurt.
Then again Korra is not very good at explaining things or even diplomacy.. she sort of expected every Spirit to go "Hell yeah.. let's kick some Kuvira ass!" just because she asked them too and was shocked that she got shot down.
She really had no arguments to speak of and expected help.. Korra needs some serious diplomacy training.
Bolin is once again my hero (and my favorite character) because as clumsy as he is his heart is in the right place and he wants to do the right thing. Kuvira abused that and misled him but Bolin wised up. Now he risks his own life for a person he had little contact with just because it is right.. i'd have been pissed if Opal still would have left him dangling.
The fight between Kuvira and Suyin was so awesome.. i actually went "Woah!" when Suying improvised the armor out of that sheet of metal.. that's some really creative usage of metal bending (and in the middle of the fight no less)
I am not that familiar with the original Avatar series so maybe (probably) i am missing much about Toph but she comes off as a real ass to Lin when she casually steps all over Lin's feelings and doesn't even realize it. Lin's outburst was totally justified but then again we saw Toph getting affected by it and realizing her error.
Yes, the fight ended abruptly but then again it was a good move because they were in the middle of enemy territory, with Kuvira present (and near effortlessly defeating Suyin) and Toph all but expended her power so chances would have been high that they would be captured again.
The real reason though (most likely) is that the final fight is reserved for Korra and Kuvira.