In that case we need some other way for Nine to be separated from Hotaru.
Bleh, what I meant by that wasn't that she couldn't
possess it, but that the level of power M9 would have through the device would be Mistress 9's power, not Hotaru's much higher level.
While I don't believe Mistress 9 is in fact a daimon, if you
do operate with her as such, you could have her literally animate the device - but combat-wise, she'd be relying on massive bursts from Hotaru's stock of cartridges (which, if she got to making her own, could be absolutely insane - Nanoha considered herself to be 'low on ammo' when she was down to three clips, eighteen bursts of power). Only way an M9-possessing-Device could be a direct threat.
As the final battle looms ever closer, the DBs search frantically for a host with sufficient magical power for Nine to use, and by some twist of fate, they stumble upon Usagi.
Oh... my.
I imagine Serenity would have a few things to say about being posessed, but this might just turn into a mirror image of the S finale except Usagi - who would not have her experiences as Sailor Moon to draw strength from - lacks the strength of character needed to break Nine's hold on her, and I'm not fond - to say the least - of fanfics with dark endings and bittersweet victories.
That mirror image idea really is interesting, though...
And you're right, Usagi probably wouldn't break free on her own. Even if you theorized she
could, as a writer you'd be better off if she
couldn't - because if she can, it really is a mirror image, and you're not going into any new and interesting territory.
But, don't let go of it just yet. (And wait, why the heck is Usagi dying a dark ending? She's not the main character, heck, as far as this story goes she's barely relevant) There ought to be a way to work up a solution.
Or the reverse of my 'possess the device' idea; Nine is expelled from Hotaru's body and finds a new host, but through a lingering connection to Hotaru, Nine is able to utilize some of Saturn's powers while crippling Hotaru's ability to fight in Senshi form.
Hm, could work. Though, why not say instead of 'lingering connection', part of her biomass was literally left in Hotaru's body, not properly removed. If M9 can connect to her separated chunk, then she can
simultaneously face directly, and perform that kind of possession/hijack/disruption.
There is no problem that cannot be solved through the proper application of immense levels of firepower.
- Finally promoted to Spammaster Indeterminate Rank as of June 18, by Stratagemini
<Stratagemini> My Titanium Anus Armour will repel all challengers!
Would you believe this is one of the more tame bits of dirt I've got for him?