Eh? You may need to get your hands on the english patch...
But would any other Servant work for this plan? I mean, Assassin was a special case in the 5th war because he was a 'false servant' summoned by Caster, so killing him would allow True Assassin, the legitimate Assassin servant, to be summoned. Had any other servant been killed, no summoning would have been possible from that servant's remains on account of their class having already been summoned and subsequently defeated. Hence killing Assassin before Zoken is able to obtain his remains prevents Zoken from summoning True Assassin.
Assassin is an oddball class to begin with. According to 'Fate/Zero', you can summon one by merely changing one of the verses of your summoning incantation. No catalyst is actually necessary as no matter what you will receive one of the Hassans regardless. But I do see your point.
Impossible...or improbable? Sorry if I'm going on a tangent here, but just exactly what does the process of inserting worms into a host entail exaclty? How is it instered? Besides Zoken's commands, what are other possible triggers that affects the activation of the worms, etc?
Think of it as a worm eating it's way into an apple... or maggots festering within a corpse... and well... multiply that sensation by hundreds... or even thousands. And then repeat the process once every few days. Fun fun fun...
So far from what I can tell the worms may active through several ways...
-Zouken can activate the worms on a whim. Anywhere.
-They go wild when exposed to aphrodisiacs.
-Dark emotions will stir them
-Lack of Prana to absorb will activate the worms as a survival instinct.
-There's also a hidden condition that Zouken implanted into Sakura's worms. 'If she participates in the Holy Grail War, she has to fight to win. Otherwise the worms will kill her.'
Having Shinji fight in her place in the other routes was something of a work around and she doesn't have to win but she's not allowed to quit either. Otherwise it triggers the worms.