Even if the Alliance should win this battle, it shouldn't be an easy victory. We NEED losses. A bittersweet victory makes for a much better story. And I don't care if the skepticism of the current deaths is kind of making these moments less effective, if they're real deaths they'll elevate the story on a reread. Which is good enough for me.
If we're getting into the specifics, I think what's most important is that there needs to be a replacement + we have some "duos" in the group - one dying and one surviving would be the most dramatic thing to do.
Kiku dies - Izo survives and rejoins the Scabbards
Inuarashi dies - Nekomamushi survives (or vice versa) - Carrot or Wanda becomes a Scabbard?
Kinemon dies - Denjiro survives and becomes the new leader - Hiyori becomes a Scabbard?
Raizo dies - Shinobu officially becomes a Scabbard
Ashura Doji dies - Hyogoro becomes a Scabbard (bandit replaced by Yakuza... that one is the least likely)
Kawamatsu dies - Gyukimaru/Onimaru becomes a Scabbard
Kanjuro dies - Yamato becomes a Scabbard (also very unlikely, but in case he doesn't join the Strawhats, this is another suitable path)
I doubt all of that will happen, just wanted to showcase that every situation might have already been accounted for.