Originally Posted by
chizLemons
Although I do very much enjoy the idea of G'raha Tia being madly in love with the WoL, that's not the canon reason. It's clear that his admiration of the WoL as a hero played a huge part because of the amount of hope and trust he had on them, and he did personally want to save the WoL. So much so that he was willing to sacrifice himself in the end, AFTER saving the First - which no, he didn't want to save for any personal connection at first, but as explained in the MSQ, it's an important step to stopping the 8th Umbral Calamity: he had to stop the rejoining. But it wasn't a decision he made alone. They decided it was worth the risk, if not for them, then for another timeline, where the calamity wouldn't happen. It's in that story.
Also, the Crystal Tower isn't presented as an easy to use time machine that can go back and forth at will anytime he wants. He arrived in the First after the Flood of Light already happened, and I think it's been also mentioned that they didn't have control of when exactly would he arrive there, considering how time flows differently. He just needed to arrive before the death of the WoL. His plan was based on what he knew from the Source, because there was no way he had any knowledge about the First other than knowing about the rejoining.