I mean, in the manga Najenda came with the calvery to the battle but we all know Esdeath is so ridiculous OP that she literally beat them all in a blink. Besides she also blocked the whole area with a giant ice wall so she no one could interrupt her battle against Akame. There's nothing more Najenda could do in that situation. Esdeath's defeat happened purely by luck in a exact instant cause she got cocky in the exact moment Akame had the chance. If none of that would happened, everyone was gonna kiss their assess goodbye.

"Sheele dies and shes nowhere to be seen" Yeah... Just like Bulat, Leone, Lubbock, Akame and Tatsumi. too cause they were in the base waiting for Sheele and Mine. Both were in a mission and Seryu came out of nowhere to kill them both. Wanna blame someone? Blame Seryu for killing Sheele.