Like you said, this isn't reality. In the game, the Arbiter Generals are the top level of martial ability due to their Blessings by Lan (Jing Yuan's Lightning Lord equivalent). That power means they're obligated to lead in the front. Because otherwise, your side is fighting with one hand purposefully tied behind your back.

And as for the decision for using Fanghu as ground zero for the nuke, again, as mentioned, it was because they were losing the war. So the alternative was losing all of the Fanghu to the Denizens of Abundance, rather than just 1/5 to the nuke.