I think some people misunderstood Nale's reasoning for joining Odium's forces. I kinda missed it too at first, but it's not that he never knew that the singers were the orginal owners of the Roshar, he and the other heralds have always known. The Stormfather even says that the truth was well known among the radiants at some point. Nale's view of justice is very feudal and he bases government legitimacy on divine right. Before he could ignore the fact that Roshar belongs to the singers, because Honor supported the humans. Now that Honor is gone, Odium is the #1 authority on Roshar (according to Nale by right of conquest). The same way a conqueror/Usurper can impose laws on his new subjects a god can do the same and impose his will. This is why Nale worked so hard to stop the desolation from starting again, now that Odium and the singers have returned all human governments (that dont side with odium) are officially illegitimate. He knew that he wouldnt be able to fight for the humans without contradicting his previous oaths as a skybreaker. However, at the same time he realises that he isnt the same as he once was and that, even if appearing logical, his reasoning may be flawed. This is why he gives Szeth the choice of swearing his 3rd Ideal to Dalinar.
Nale's decision here makes a lot of sense if you agree with intial premises. And I think it really highlights the flaws in Honors shard in following bonds and oaths for the sake of it even when it ends up undermining the oaths original purpose.
Personally I'm with Nighblood on this
''Don't be stupid, lets go fight those guys''.