There is nothing Sephiran can do to stop the Begnion royalty from enslaving Laguz and otherwise staying at odds with the Laguz. Beorc and Laguz had already broken the vow renouncing war in the first place, so no matter what happened, Ashera was going to judge the world the exact same way she ends up doing in RD anyways. Lehran's entire job was to sing the Galdr of Release so that Ashera and Yune could talk before Ashera decides to pass her judgment, but he could no longer do it because he was stripped of his powers when he lay with Altina. He resided in Goldoa for 700 years watching the Laguz be enslaved and when he finally found his Granddaughter, she was promptly murdered for being Branded. Then his people were wiped out. He lost EVERYTHING.
Yes he could. He was literally already publicly opposing them in the games themselves. Doing something like covertly poisoning them or suddenly have the Black Knight or Ashnard massacre them or just teaming up with the other laguz to attack Begnion's senate is a MUCH easier solution than his convoluted plan to wipe humanity that took more than a decade to plan. And again, there was no war at all, let alone a global one until Sephiran himself created one. It's not just Beorc and Laguz renouncing war, it had to be a GLOBAL one, which is why Yune and Ashera don't wake up until Kurthnaga as representative of the Dragons went in. Yes, him being torn with grief is understandable, but he literally aids the villains who enslaved and killed his people. It's like if a Jew, in revenge for the Holocaust and anti-Semites still all over the world, decided to team up with Neo-Nazis to nuke the world. You're pulling mental gymnastics to defend the sloppy writing. If I tried, I could defend Fate's writing the way you are as well.