[Akiyoshi]

Olivie as Saber? Guess that will finally close the full circle of expiness between her and Arturia xD!

Considering there's a giant bunch of grimderp fanfiction about Nanohaverse characters and blending with verses full of death and misery (and fanservice) that MOST of the fandom seems to love i really don't see a problem devising a motivation for some of the Nanoha characters to fight a war. Hell, the Wolks are practically tailor-suited for the task. Tragic Bakcstory? Check, Experience in murdering? Check, capacity to fail horribly? ...Check (thanks FORCE xDU). They even have the medieval fantasy motif xD!

Of course if you doesn't want to be so cliche you can summon another guy ....say Zest! He hits every trope you mentioned including the part of being dead xD!